repo: install package repositories in "arbitrary" paths

[tigarmo]

This commit adds a new public function, "install_in_root()", that installs package repositories in a location that contains an Apt-based system, and not the host system. The intention is that eventually said system will be chroot'ed into, at which point an "apt update" will pick up the new repositories.

The motivation is to support package repositories in overlays in Rockcraft.

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

---

[codecov[bot]]

Codecov Report

Merging #80 (0bae11f) into main (8e238f0) will decrease coverage by 0.31%. The diff coverage is 75.75%.

:exclamation: Current head 0bae11f differs from pull request most recent head 2673f7a. Consider uploading reports for the commit 2673f7a to get more accurate results

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   88.17%   87.86%   -0.31%     
==========================================
  Files          13       13              
  Lines         634      659      +25     
  Branches      125      131       +6     
==========================================
+ Hits          559      579      +20     
- Misses         61       66       +5     
  Partials       14       14              

Impacted Files	Coverage Δ
craft_archives/repo/installer.py	48.88% <27.27%> (-3.75%)	:arrow_down:
craft_archives/repo/__init__.py	100.00% <100.00%> (ø)
craft_archives/repo/apt_key_manager.py	98.29% <100.00%> (+0.07%)	:arrow_up:
craft_archives/repo/apt_preferences_manager.py	87.91% <100.00%> (+0.70%)	:arrow_up:
craft_archives/repo/apt_sources_manager.py	93.93% <100.00%> (+0.53%)	:arrow_up: