Add support for user-defined build settings in external repositories

[chsigg]

When trying it all out, --//:myflag=foo works, but --@rules_my_language//:myflag=foo does not. I've tried to make an alias and then go for the first alternative, but that fails as well with ERROR: Unrecognized option: //:myflag.

Is it a design choice to not allow command line flags from external dependencies? Is a bug so I should file a bug report? Is it a feature request to allow it in v2, possibly via the WORKSPACE file?

Originally posted by @moroten in #5577 (comment)

[moroten]

My example in #10499 exercises both --//:myflag=foo and --@rules_my_language//:myflag=foo. Which Bazel version are you using?

The problem in #10499 is that @rules_my_language//:myflag and //:myflag are not mapped to the same instance, so you end up in errors you would not expect. As a workaround, I do never define any build setting in the root workspace.

[chsigg]

That's wonderful, thanks for the info. Indeed, I got tripped by trying this out in the root workspace. When referencing from another repo, it works as expected. That is the actual use case for me. Closing.