Remove the $libdir/ prefix from module_pathname

[theory]

It limits the search to PKGLIBDIR; if anyone installs it into another directory in dynamic_library_path, Postgres won't be able to find it.

Details.

I think pgrx should also consider adding something equivalent to the PGRX prefix param to allow the sharedir and pkglibdir files to be installed somewhere other than where pg_config suggests to properly support dynamic_library_path and the new extension_control_path configs.

[eeeebbbbrrrr]

This is probably just fine to merge, so I'll go ahead and do that.

I'm not up-to-date on dynamic_library_path and extension_control_path, so I'll have to do some reading. What did you have in mind, @theory?

[theory]

Right now pgrx exclusively installs shared libraries in PKGLIBDIR and control files and SQL file sin SHAREDIR. What it needs, I think, is a --prefix option that installs them under a different prefix. The analog is the PGXS prefix= option, documented here (search for "You can select a separate directory prefix"). For better or worse, it should probably have the same behavior, adding postgresql to the end if neither postgres nor pgsql appear in the value. Then pgrx should append share for the sharedir, share/extensions for the control and SQL files, share/$dir if the Control file directory option is specified. It should append lib for pkglibdir. Presumably doc files would go under $prefix/docs.