install: Support local paths/uris

If the remote is a local path (absolute or relative starting with ./)
then we convert this to a file: uri, which are now supported to
install directly from a local repo.

This is very useful when testing locally built apps.

Closes: #1244
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f