Collapse absolute globs to watch relative paths in source configs

[rnons]

Description of the change

I often have a setup like this

├── examples
└── src

in examples/spago.dhall, I have

, sources =
    [ "src/**/*.purs", "../src/**/*.purs" ]

The problem is file changes in the root src folder won't trigger rebuild, because /home/rnons/pkg/src/a/b.purs doesn't match /home/rnons/pkg/examples/../src/**/*.purs.

This PR collapses /home/rnons/pkg/examples/../src/**/*.purs to /home/rnons/pkg/src/**/*.purs, so that watching works as expected.

Checklist:

• Added the change to the "Unreleased" section of the changelog

[f-f]

Looks great, thank you! 🙂