Inter-repo dependencies

[leto]

If repo A depends on the latest commit from repo B, tests will fail, because, by default, each test suite run only knows what is in PERL5LIB.

If git repos are being reused, then we can add the optional feature where /tmp/build/sgn/repoX/lib is added to PERL5LIB.

This is a perl-specific feature, but something similar should be possible for other languages.