cmd/dist: add a test checking that only approved modules are vendored

[jba]

Add a test to the cmd/dist package that compares the modules in src/vendor and src/cmd/vendor against a list of allowed module globs.

[gopherbot]

Change https://go.dev/cl/564355 mentions this issue: src/cmd/dist: add check for allowed vendor modules

[jba]

This was fixed. Closing.