This was previously in runtime, because pam_unix uses it and we don't
want to pull in the entire yp package as a dependency of runtime.

However, we can now use LIB_PACKAGE here to create a yp-lib package
to contain the library, which is a much more reasonable dependency.
Since libypclnt clearly belongs in the yp package, move it there.

This change moves files between packages so, until we have a proper
policy on how to handle this in release/stable branches, it should
not be MFC'd.

MFC after: never