Providers aren't shareable across different workspace aliases

[endobson]

Given a workspace with name my_workspace, I can load .bzl files using the paths //path/to:file.bzl or @my_workspace//path/to:file.bzl. I would expect these to be the equivalent, but they are not. It seems that providers are distinct between the two paths, which leads to awful error messages because there is no distinction in the error message between the two providers.

endobson@yggdrasil () ~/tmp/example-repo % bazel build //:all                                                                                                                                                                             (1)
INFO: Reading 'startup' options from /Users/endobson/.bazelrc: --watchfs
ERROR: /Users/endobson/tmp/example-repo/BUILD:30:10: in deps attribute of my_rule rule //:base3: '//:remote1' does not have mandatory providers: 'my_provider'.
ERROR: /Users/endobson/tmp/example-repo/BUILD:25:10: in deps attribute of my_rule rule //:remote3: '//:base1' does not have mandatory providers: 'my_provider'.
WARNING: errors encountered while analyzing target '//:base3': it will not be built.
WARNING: errors encountered while analyzing target '//:remote3': it will not be built.
INFO: Analysis succeeded for only 4 of 6 top-level targets
INFO: Found 4 targets...
ERROR: command succeeded, but not all targets were analyzed.
INFO: Elapsed time: 0.139s, Critical Path: 0.00s

endobson@yggdrasil () ~/tmp/example-repo % bazel version                                                                                                                                                                                  (1)
INFO: Reading 'startup' options from /Users/endobson/.bazelrc: --watchfs
Build label: 0.5.4- (@non-git)
Build target: bazel-out/darwin_x86_64-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Thu Aug 31 04:32:51 2017 (1504153971)
Build timestamp: 1504153971
Build timestamp as int: 1504153971

Example available at:
https://github.com/endobson/examples/tree/bazel-provider

[dslomov]

Duplicate of #3493