fix ignoring signatures on remote repos with only InRelease file

[neolynx]

Fixes #237

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

If a remote repo has only InRelease but no Release file, aptly is not able to mirror it, even when -ignore-signatures is specified.

This change fixes this behavior, by trying to download the InRelease if Release file is not found, and strips and ignores the signature.

Checklist

• functional test added/updated (if change is functional)

[codecov]

Codecov Report

Attention: Patch coverage is 80.35714% with 11 lines in your changes missing coverage. Please review.

Project coverage is 74.47%. Comparing base (4661913) to head (0f5f648).

Files	Patch %	Lines
deb/remote.go	47.36%	6 Missing and 4 partials ⚠️
pgp/gnupg.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1308      +/-   ##
==========================================
- Coverage   74.53%   74.47%   -0.06%     
==========================================
  Files         146      146              
  Lines       16576    16597      +21     
==========================================
+ Hits        12355    12361       +6     
- Misses       3245     3257      +12     
- Partials      976      979       +3     

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