another possible setup bug: not accepting "provides" name in -P

[Jeremy Drake]

I was trying to clean up my workflows now that llvm 20.1.8-2 is published,
and I came across another possible setup bug.  It seems that the -P option
doesn't accept a package name that is provided rather than being the
actual name of a pacakge.

llvm20.1-devel is provided by llvm-devel 20.1.8-2, but passing that to -P
results in the message: Package 'libllvm20.1-devel' not found.  However,
libclang-devel 20.1.8-2 requires libllvm20.1-devel, and selecting that
version results in "Install libllvm-devel 20.1.8-2 (automatically added)",
so requires does respect provides.

This would be a problem for scallywag, which I believe feeds
BUILD_REQUIRES straight into -P, and BUILD_REQUIRES is specified to allow
provided names in addition to acutal package names.

(A lesser issue is that cygport warns that the provided name is not
installed when the package that provides it is in fact installed, but this
is probably a limitation of the installed package database not recording
what the installed packages provide)