Support relative file system paths when Deckfile is loaded via Git

[Schille]

It is possible to load a Deckfile via Git like so deck get git@mygithost.com/repo.git#branch. This loads the Deckfile and processes it accordingly.
When there are relative file system paths included in the sources this won't work as the repo is not present on the local machine. Same with other sources.

[buschNT]

@Schille I just tested this issue on the "81" branch using the current "main" branch from getdeck:
deck get git@github.com:Getdeck/getdeck.git#81
The new deckfile at the root of the branch 81 has a local file reference which works. This issue might be solved by the refactoring we did.
Please feel free to try to reproduce this issue using other sources.

[Schille]

This is awesome.