metadata_dictionary/base: Compare metadata filenames in lowercase

SingleFileIntrinsicMapping.detect_metadata_files was comparing lowercase
versions of filenames with SingleFileIntrinsicMapping.filename variable
value to detect metadata files.

But as SingleFileIntrinsicMapping.filename holds the canonical name of a
metadata file it can contain uppercase characters.

So ensure to compare lowercase versions of both filenames to avoid metadata
files being undetected.

This fixes indexing of Python intrinsic metadata.