A cpv can be returned from dbapi.match() as an old-style virtual match even

A cpv can be returned from dbapi.match() as an old-style virtual match even
in cases when the package does not actually PROVIDE the virtual. Filter out
any such false matches inside depgraph._select_package() and
_show_unsatisfied_dep(). Thanks to Ned Ludd <solar@g.o> for reporting this
issue which was discovered when attempting to install virtual/libc on uclibc
profile. Apparently the uclibc ebuilds need to be fixed to properly set
PROVIDE when the metadata cache is generated.

svn path=/main/trunk/; revision=11622