resourceloader: Use absolute file path in SpecialCharactersDataModule

resourceloader: Use absolute file path in SpecialCharactersDataModule

Follows-up ec2330f.

When called, it fails fails:
> file_get_contents(resources/src/mediawiki.language/specialcharacters.json):
> failed to open stream: No such file or directory

Needs to be expanded relative to $IP, just like ResourceLoaderFileModule
and others do already.

Not previously caught by unit tests because this method was never
called from any tests.

Change-Id: Id87b3de29a61039d87b630202f1f71f112cbd752