cubicweb-vcsfile #2794142 clarify local_cache handling wrt path value [validation pending]

As of today, a repo with a source_url will get a "local cache" clone, but a repo with a "path" (reachable through the filesystem) has none.

The original use case for "path" => no local_cache may have been a misguided attempt at disk space optimisation. I believe the local_cache should always be there.

done in2.0.0
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