Go modules internals

The internals of Go modules, extracted from golang/go. All occurrences of internal in import paths have been rewritten to _internal_, which makes these packages importable, but everything is otherwise unchanged.

Re-extracting

The extract/ directory contains the code to do this. Run the Go program, then copy the contents of the out/github.com/dependabot/gomodules-extracted directory to the top level of this repo.