Provide a target substitution mechanism (e.g., in com/google/dagger let deps on :dagger dep on :dagger_api)

[cgruber]

Given that we allow build segment rewriting per-artifact, to allow things like the dagger dep to carry an exported_plugin, but also because we are about to have automated dependency discovery from pom files, this leads to odd cases. For instance, where (say), our wrapped :dagger target exports :dagger_plugin as an exported_plugin, and that depends on :dagger_compiler, the inferred deps list (from the pom) of :dagger_compiler would naively depend on :dagger. This creates a cycle. At present, we can verbosely just replace any build snippets that are affected, it becomes gross for something like dagger, where all the other :dagger-* packages depend on the core :dagger apis.

Having a mechanism to, for instance, say "for all of com/google/dagger/BUILD" any inferred deps on com.google.dagger:dagger should reference :dagger_api instead of :dagger we can specify these surgical changes more tersely.

I think it only makes sense to do these substitutions on a per-build-file basis, because it's really only to address these intra-BUILD situations, and we don't want ALL things to depend on :dagger_api, because it's only the implementation details we need to swap. We created the :dagger wrapper target precisely to enrich it, so we don't want to steer other consumers of dagger to the naive jar that doesn't have the exported_plugin.

So something like:

maven_repository_specification(
    name = "maven",
    artifacts = [ ... ],
    build_substitutions = {
        "com.google.dagger:dagger": DAGGER_TARGET_REPLACEMENT_SNIPPET,
    },
    target_substitutions = {
        "com.google.dagger": { "dagger": "dagger_api" }
    },

This means that, in the context of this group (and the package @maven/com/google/dagger and it's BUILD file) treat any dependency on dagger to be a dependency on dagger_api.