pgsql: Fix: Don't strip $libdir from nested module_pathnames

Fix: Don't strip $libdir from nested module_pathnames

This patch fixes a bug in how 'load_external_function' handles
'$libdir/ prefixes in module paths.

Previously, 'load_external_function' would unconditionally strip
'$libdir/' from the beginning of the 'filename' string. This caused
an issue when the path was nested, such as "$libdir/nested/my_lib".
Stripping the prefix resulted in a path of "nested/my_lib", which
would fail to be found by the expand_dynamic_library_name function
because the original '$libdir' macro was removed.

To fix this, the code now checks for the presence of an additional
directory separator ('/' or '\') after the '$libdir/' prefix. The
prefix is only stripped if the remaining string does not contain a
separator. This ensures that simple filenames like '"$libdir/my_lib"'
are correctly handled, while nested paths are left intact for
'expand_dynamic_library_name' to process correctly.

Reported-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Co-authored-by: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Co-authored-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CAFiTN-uKNzAro4tVwtJhF1UqcygfJ%2BR%2BRL%3Db-_ZMYE3LdHoGhA%40mail.gmail.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d07e2d4237cb2972d396a5b1f338457db9a21a2c

Modified Files
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src/backend/utils/fmgr/dfmgr.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)