go_library()'s import path can now be customized.

Addresses bazel-contrib#16.

examples/bin/BUILD

@@ -12,3 +12,12 @@ go_binary(
         "//examples/vendor/github_com/user/vendored:go_default_library",
     ],
 )
+
+go_binary(
+    name = "bin_with_rename",
+    srcs = ["bin.go"],
+    deps = [
+        "//examples/lib:go_default_library",
+        "//examples/vendor/github_com/user:vendored",
+    ],
+)

examples/vendor/github_com/user/BUILD

@@ -0,0 +1,13 @@
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+load("//go:def.bzl", "go_library")
+
+go_library(
+    name = "vendored",
+    srcs = [
+        "vendored_without_buildfile/vendored.go",
+    ],
+    source_import = "github_com/user/vendored",
+)

examples/vendor/github_com/user/vendored_without_buildfile/vendored.go

@@ -0,0 +1,21 @@
+/* Copyright 2016 The Bazel Authors. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package vendored
+
+// Vendored returns a string.
+func Vendored() string {
+	return "I was vendored"
+}

go/def.bzl

@@ -160,6 +160,9 @@ def emit_go_compile_action(ctx, sources, deps, out_lib, cgo_object=None):
     if source_import.rfind(_VENDOR_PREFIX) != -1:
       source_import = source_import[len(_VENDOR_PREFIX) + source_import.rfind(_VENDOR_PREFIX):]
 
+    if len(d.source_import):
+      source_import = d.source_import
+
     if source_import != actual_import:
       if source_import in import_map:
         fail("duplicate import %s: adding %s and have %s"
@@ -252,6 +255,7 @@ def go_library_impl(ctx):
     transitive_go_library_object = transitive_libs,
     cgo_object = cgo_object,
     transitive_cgo_deps = transitive_cgo_deps,
+    source_import = ctx.attr.source_import,
   )
 
 def _c_linker_options(ctx, blacklist=[]):
@@ -443,6 +447,7 @@ go_library_attrs = go_env_attrs + {
     "library": attr.label(
         providers = ["go_sources", "cgo_object"],
     ),
+    "source_import": attr.string(),
 }
 
 go_library_outputs = {