Handle project that uses rules_spm and one of its dependencies also uses rules_spm

[cgrindel]

Project A - Has dep on a package (e.g. swift-nio).
Project B - Has dep on Project A and a package that Project A uses as a dep (e.g. swift-nio)

Problem:

1. If spm_repositories for Project A and B have the same name, we will get a duplicate declaration.
2. If spm_repositories for Project A and B have the same names, we will get duplicate symbol errors during compile/linking

Options:

1. Require that Project A provides a Package.swift; Project B would use spm_repositories to use the dependency.
2. Rework rules_spm deps logic;

• Each project that uses rules_spm would provide their package info (i.e., deps and platforms) via function.
• The root project would collect all of the package info for all of the deps (and deps of deps) and pass it to spm_repositories.

Option 1 Thoughts

• Biggest benefit: SPM figures it all out; it already handles all of the dep merging mechanics
• We could provide a build rule to generate a Package.swift; it could be copied to source using rules_updatesrc.
• Non-Bazel projects can consume the project
• Biggest downside: Can't treat the dep repository as a Bazel repo (i.e., can't use local_repository)
  • Note: spm_pkg does support local packages. So, this may not be a big deal.

Option 2 Thoughts:

• Biggest benefit: Can treat dep repository as a Bazel repo
• Biggest downside: Need to rework package declarations
  • Each project that uses rules_spm would implement a function that returns their package info (deps and platforms)