shared/install: when looking for symlinks in .wants/.requires, ignore symlink target

shared/install: when looking for symlinks in .wants/.requires, ignore symlink target

We'd say that file is enabled indirectly if we had a symlink like:
  foo@.service ← bar.target.wants/foo@one.service
but not when we had
  foo@one.service ← bar.target.wants/foo@one.service

The effect of both link types is the same. In fact we don't care
about the symlink target. (We'll warn if it is mismatched, but we honour
it anyway.)

So let's use the original match logic only for aliases.
For .wants/.requires we instead look for a matching source name,
or a source name that matches after stripping of instance.