Before this patch, plugin units were linked with the target code even
when the unit was passed via `-plugin-package`. This is an issue to
support plugins in cross-compilers (plugins are definitely not ABI
compatible with target code).

We now clearly separate unit dependencies for plugins and unit
dependencies for target code and only link the latter ones.

We've also added a test to ensure that plugin units passed via
`-package` are linked with target code so that `thNameToGhcName` can
still be used in plugins that need it (see T20218b).