Fix/mirror download already downloaded

[neolynx]

Fixes #1194

might fix #1289 ?

Description of the Change

mirror: do not download already downloaded packages

this change imports downloaded packages into the pool immediately after download.
in case mirroring is aborted and later resumed, already downloaded packages will not be downloaded anymore.

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 74.86%. Comparing base (b4cd86a) to head (254e0a7).

Files	Patch %	Lines
api/mirror.go	43.75%	7 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1296 +/- ## ======================================= Coverage 74.86% 74.86% ======================================= Files 144 144 Lines 16268 16261 -7 ======================================= - Hits 12179 12174 -5 + Misses 3150 3149 -1 + Partials 939 938 -1 ```

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