Add fast path for CVS dep

The search mechanism in MacOS::Xcode is very slow. It requires shelling
out at least twice, and possibly a third time (in the CLT-only case).

Calling provides_cvs? activates this in order to determine the Xcode
version. But if we know that there isn't an Xcode available for the
current OS that meets the criteria, we can avoid this check entirely.
Remove special behavior of autotools symbol deps