Don't strip $libdir from LOAD command

Don't strip $libdir from LOAD command

Commit 4f7f7b03758 implemented the extension_control_path GUC, and to
make it work it was decided that we should strip the $libdir/ on
module_pathname from .control files, so that extensions don't need to
worry about this change.

This strip logic was implemented on expand_dynamic_library_name()
which works fine when executing the SQL functions from extensions, but
this function is also called when the LOAD command is executed, and
since the user may explicitly pass the $libdir prefix on LOAD
parameter, we should not strip in this case.

This commit fixes this issue by moving the strip logic from
expand_dynamic_library_name() to load_external_function() that is
called when the running the SQL script from extensions.

Reported-by: Evan Si 
Author: Matheus Alcantara 
Reviewed-by: Nathan Bossart 
Reviewed-by: Rahila Syed 
Bug: #18920
Discussion: https://www.postgresql.org/message-id/flat/18920-b350b1c0a30af006%40postgresql.org