Recompilation checking for home package plugins is not transitive enough

Similar setup to #19771 (closed) but this time with plugins

https://github.com/mpickering/jubilant-barnacle

If a HPT plugin depends on a library, we also need to recompile when the libraries object files change.

In this example modifying p/Lib.p should cause the plugin to recompile and hence reapplied to Q.Main but the recompilation checker doesn't do this which leads to incorrect results.