Doing this by performing string interpretation on the repo name is a bit
brittle. And it works less well under Bzlmod where the unpinned repo
name is going to look something like
`rules_jvm_external~~maven~unpinned_maven`.

After this change, the lockfile pinning script, `@maven//:pin` will use the local
machine's Coursier cache (`~/Library/Caches/Coursier` on MacOS) even under
Bzlmod. A populated cache makes version resolutions substantially faster to obtain.

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