Support for cross-repo code navigation

[varungandhi-src]

This should mostly "just work" with SCIP, but we haven't tested packages yet. We should fix this after completing:

• ☂️ Functionality for v0.2 #1
• Investigate if we can do best-effort indexing with # typed: false #30

High-level breakdown of the work involved here:

• Support manually specifying file to gem mappings. feat: Add support for manually specifiying gems for cross-repo. #149
• Support inferring file to gem mappings from standard Sorbet filesystem layout. feat: Infer gem info from standard filesystem layout. #155
• feat: Try to read dependency versions from Gemfile.lock. #158
• Support inferring file to gem mappings from standard Bundler paths (for projects not using Sorbet).
• 🐛 Incorrect package names for references #157
• Emit external symbols in external_symbols list (not 100% sure if this is needed)
• Specially add Core and Standard APIs to Sourcegraph.
• Additional manual testing against Sourcegraph + refinement.

[varungandhi-src]

Related:

• https://github.com/sourcegraph/sourcegraph/issues/44122