Don't import rules with a conflicting definition.

This avoids manual `excludes=[]` and `overrides={}`, which were suggested in the linked issues.  We exclude things that have already been imported, so to override them, simply import them before `docker_repositories()` and the exclusion logic will handle the rest.

Fixes: bazelbuild#55
Fixes: bazelbuild#77

docker/docker.bzl

@@ -32,53 +32,62 @@ CONTAINERREGISTRY_RELEASE = "v0.0.11"
 
 def docker_repositories():
   """Download dependencies of docker rules."""
-  native.http_file(
-    name = "puller",
-    url = ("https://storage.googleapis.com/containerregistry-releases/" +
-           CONTAINERREGISTRY_RELEASE + "/puller.par"),
-    sha256 = "90a76d01ee57a5df7353a533c96966822c9efda61636c11eac11344171556017",
-    executable = True,
-  )
+  excludes = native.existing_rules().keys()
 
-  native.http_file(
-    name = "pusher",
-    url = ("https://storage.googleapis.com/containerregistry-releases/" +
-           CONTAINERREGISTRY_RELEASE + "/pusher.par"),
-    sha256 = "1989ceb41144784dccb3476cea5b5f1bef112bb0aae56544b9c56572894c38ab",
-    executable = True,
-  )
+  if "puller" not in excludes:
+    native.http_file(
+      name = "puller",
+      url = ("https://storage.googleapis.com/containerregistry-releases/" +
+             CONTAINERREGISTRY_RELEASE + "/puller.par"),
+      sha256 = "90a76d01ee57a5df7353a533c96966822c9efda61636c11eac11344171556017",
+      executable = True,
+    )
 
-  native.git_repository(
-    name = "containerregistry",
-    remote = "https://github.com/google/containerregistry.git",
-    tag = CONTAINERREGISTRY_RELEASE,
-  )
+  if "pusher" not in excludes:
+    native.http_file(
+      name = "pusher",
+      url = ("https://storage.googleapis.com/containerregistry-releases/" +
+             CONTAINERREGISTRY_RELEASE + "/pusher.par"),
+      sha256 = "1989ceb41144784dccb3476cea5b5f1bef112bb0aae56544b9c56572894c38ab",
+      executable = True,
+    )
+
+  if "containerregistry" not in excludes:
+    native.git_repository(
+      name = "containerregistry",
+      remote = "https://github.com/google/containerregistry.git",
+      tag = CONTAINERREGISTRY_RELEASE,
+    )
 
   # TODO(mattmoor): Remove all of this (copied from google/containerregistry)
   # once transitive workspace instantiation lands.
-  native.new_http_archive(
-    name = "httplib2",
-    url = "https://codeload.github.com/httplib2/httplib2/tar.gz/v0.10.3",
-    sha256 = "d1bee28a68cc665c451c83d315e3afdbeb5391f08971dcc91e060d5ba16986f1",
-    strip_prefix = "httplib2-0.10.3/python2/httplib2/",
-    type = "tar.gz",
-    build_file_content = """
+  if "httplib2" not in excludes:
+    # TODO(mattmoor): Is there a clean way to override?
+    native.new_http_archive(
+      name = "httplib2",
+      url = "https://codeload.github.com/httplib2/httplib2/tar.gz/v0.10.3",
+      sha256 = "d1bee28a68cc665c451c83d315e3afdbeb5391f08971dcc91e060d5ba16986f1",
+      strip_prefix = "httplib2-0.10.3/python2/httplib2/",
+      type = "tar.gz",
+      build_file_content = """
 py_library(
    name = "httplib2",
    srcs = glob(["**/*.py"]),
    data = ["cacerts.txt"],
    visibility = ["//visibility:public"]
 )""",
-  )
+    )
 
   # Used by oauth2client
-  native.new_http_archive(
-    name = "six",
-    url = "https://pypi.python.org/packages/source/s/six/six-1.9.0.tar.gz",
-    sha256 = "e24052411fc4fbd1f672635537c3fc2330d9481b18c0317695b46259512c91d5",
-    strip_prefix = "six-1.9.0/",
-    type = "tar.gz",
-    build_file_content = """
+  if "six" not in excludes:
+    # TODO(mattmoor): Is there a clean way to override?
+    native.new_http_archive(
+      name = "six",
+      url = "https://pypi.python.org/packages/source/s/six/six-1.9.0.tar.gz",
+      sha256 = "e24052411fc4fbd1f672635537c3fc2330d9481b18c0317695b46259512c91d5",
+      strip_prefix = "six-1.9.0/",
+      type = "tar.gz",
+      build_file_content = """
 # Rename six.py to __init__.py
 genrule(
     name = "rename",
@@ -91,16 +100,18 @@ py_library(
    srcs = [":__init__.py"],
    visibility = ["//visibility:public"],
 )"""
-  )
+    )
 
   # Used for authentication in containerregistry
-  native.new_http_archive(
-    name = "oauth2client",
-    url = "https://codeload.github.com/google/oauth2client/tar.gz/v4.0.0",
-    sha256 = "7230f52f7f1d4566a3f9c3aeb5ffe2ed80302843ce5605853bee1f08098ede46",
-    strip_prefix = "oauth2client-4.0.0/oauth2client/",
-    type = "tar.gz",
-    build_file_content = """
+  if "oauth2client" not in excludes:
+    # TODO(mattmoor): Is there a clean way to override?
+    native.new_http_archive(
+      name = "oauth2client",
+      url = "https://codeload.github.com/google/oauth2client/tar.gz/v4.0.0",
+      sha256 = "7230f52f7f1d4566a3f9c3aeb5ffe2ed80302843ce5605853bee1f08098ede46",
+      strip_prefix = "oauth2client-4.0.0/oauth2client/",
+      type = "tar.gz",
+      build_file_content = """
 py_library(
    name = "oauth2client",
    srcs = glob(["**/*.py"]),
@@ -110,26 +121,29 @@ py_library(
      "@six//:six",
    ]
 )"""
-  )
+    )
 
   # Used for parallel execution in containerregistry
-  native.new_http_archive(
-    name = "concurrent",
-    url = "https://codeload.github.com/agronholm/pythonfutures/tar.gz/3.0.5",
-    sha256 = "a7086ddf3c36203da7816f7e903ce43d042831f41a9705bc6b4206c574fcb765",
-    strip_prefix = "pythonfutures-3.0.5/concurrent/",
-    type = "tar.gz",
-    build_file_content = """
+  if "concurrent" not in excludes:
+    # TODO(mattmoor): Is there a clean way to override?
+    native.new_http_archive(
+      name = "concurrent",
+      url = "https://codeload.github.com/agronholm/pythonfutures/tar.gz/3.0.5",
+      sha256 = "a7086ddf3c36203da7816f7e903ce43d042831f41a9705bc6b4206c574fcb765",
+      strip_prefix = "pythonfutures-3.0.5/concurrent/",
+      type = "tar.gz",
+      build_file_content = """
 py_library(
    name = "concurrent",
    srcs = glob(["**/*.py"]),
    visibility = ["//visibility:public"]
 )"""
-  )
+    )
 
   # For packaging python tools.
-  native.git_repository(
-    name = "subpar",
-    remote = "https://github.com/google/subpar",
-    commit = "7e12cc130eb8f09c8cb02c3585a91a4043753c56",
-  )
+  if "subpar" not in excludes:
+    native.git_repository(
+      name = "subpar",
+      remote = "https://github.com/google/subpar",
+      commit = "7e12cc130eb8f09c8cb02c3585a91a4043753c56",
+    )