Recompilation: Handle -plugin-package correctly

If a plugins was specified using the -plugin-package-(id) flag then the
module it applied to was always recompiled.

The recompilation checker was previously using `findImportedModule`,
which looked for packages in the HPT and then in the package database
but only for modules specified using `-package`.

The correct lookup function for plugins is `findPluginModule`, therefore
we check normal imports with `findImportedModule` and plugins with
`findPluginModule`.

Fixes #20417