Stan expects relative paths. Without this change, file names won't map
correctly to their associated language extension data, which means no
enabled extensions will be detected. This causes annoying false
positives with, e.g., the `StrictData` extension. (See issue haskell#3174.)

We specifically want to test this diagnostic, so we need it to fire.

…sions

Includes test cases for both `LANGUAGE` pragmas and extensions enabled
in a project's `.cabal` file.

These changes ensure that the tests will fail given bad mappings in
either the `cabalExtensionsMap` OR the `checksMap`. Either of these
could cause bad behavior as seen in issue haskell#3174.

The Stan plugin will still operate as expected even if we can't load a
config -- it will simply default to showing all inspections.

…tors

 Related to (what I assume is) a bug in Stan, or its `extensions`
 library. Regardless of OS, the `hs-source-dirs` field is prepended
 as-is to the module name to create the file paths used in the cabal
 extensions map. This means the maps won't work in Windows if your cabal
 file contains `/` path separators. Working around the limitation here
 to ensure tests work on all platforms.