lint: lint snap files inside an instance

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

So far, snapcraft lint parses the command line, prepares an instance, and re-executes snapcraft inside the instance.

This PR adds the code that is executed inside the instance:

1. Unsquash the snap
2. Collect the metadata from snap.yaml and lint filters from snapcraft.yaml
3. Install the snap
4. Run the linter on /snap/<snap-name>/current
5. Report the linter warnings

(CRAFT-1689)

[codecov-commenter]

Codecov Report

Merging #4115 (2a4729d) into main (70f9e9d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #4115      +/-   ##
==========================================
+ Coverage   94.78%   94.80%   +0.02%     
==========================================
  Files         648      648              
  Lines       54034    54252     +218     
==========================================
+ Hits        51214    51436     +222     
+ Misses       2820     2816       -4     

Impacted Files	Coverage Δ
snapcraft/commands/lint.py	100.00% <100.00%> (+2.94%)	⬆️
snapcraft/linters/__init__.py	100.00% <100.00%> (ø)
snapcraft/linters/linters.py	100.00% <100.00%> (+2.32%)	⬆️
tests/unit/commands/test_lint.py	100.00% <100.00%> (ø)

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[tigarmo]

Callahan I'm afraid I tried my best to find issues with your code but I failed :/
this is very clear and well-organized, very nice! The unit tests are verbose with the mocking but it's consistent with the other tests, so no issues there.
thanks!