repo: permit file URLs

[lengau]

• [x] Have you followed the guidelines for contributing?
• [x] Have you signed the CLA?
• [x] Have you successfully run tox?

---

Fixes #92

[codecov[bot]]

Codecov Report

Merging #93 (83daa4a) into main (be875b0) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   90.10%   90.10%           
=======================================
  Files          13       13           
  Lines         687      687           
  Branches      140      140           
=======================================
  Hits          619      619           
  Misses         50       50           
  Partials       18       18           

Impacted Files	Coverage Δ
craft_archives/repo/package_repository.py	98.76% <100.00%> (ø)