(#14962) PMT doesn't support setting a relative modulepath

We previously fixed expansion for the target_dir, but this only worked when the
target_dir was set explicitly, it didn't support relative paths being passed in
the modulepath. This patch fixes that and adds tests.

As a side-effect, this should also fixes cases where the first modulepath
defined in the configuration file is relative.

It also corrects the tests previously applied for expanding the target_dir, as
it used to rely on expanding the target_dir before adding it to the modulepath.
This wasn't necessary, so now we don't bother testing that the targetdir is
expanded in the modulepath after being added.

Acceptance tests have been added for testing modulepath, with absolute
and relative paths.