automatic detection of so provides checks /opt.

[Lunarequest]

I'm currently working on packaging pypy binaries to bootstrap it and I found an issue with melange's so detection. Pypy has to have certain so files relative to it eg libtinfow.so.6 and libffi.so.6 hence it is usually packaged in /opt with symlinks to bin. However melange detects these as provides as well and causes conflicts prevent building pypy from source using its binaries.

[kaniini]

We should add a list of path prefixes to skip for the automatic detection I think.