Use BUILD.bazel for third_party packages

Both `BUILD` and `BUILD.bazel` could be used as the bazel project file
and `BUILD.bazel` is preferred (see bazelbuild/bazel#4517 (comment))

This fix changes generated BUILD in third_party packages to `BUILD.bazel`.
This will help avoid conflict with `BUILD` or `build` file/directory names
in third party packages.

For example, while working on 19461 I noticed that apache thrift package
consists of a `build` directory and that causes issues in case-insensitive
systems like macOS. This PR should help avoid such conflict issues.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

repo.bzl

@@ -88,7 +88,9 @@ def _tf_http_archive(ctx):
   if ctx.attr.patch_file != None:
     _apply_patch(ctx, ctx.attr.patch_file)
   if ctx.attr.build_file != None:
-    ctx.template("BUILD", ctx.attr.build_file, {
+    # Use BUILD.bazel to avoid conflict with third party projects with
+    # BUILD or build (directory) underneath.
+    ctx.template("BUILD.bazel", ctx.attr.build_file, {
         "%prefix%": ".." if _repos_are_siblings() else "external",
     }, False)