[WIP] Support merging of registries.

[GunnarFarneback]

Functionality to merge registries or copy selected packages from one registry to another.

[codecov-commenter]

Codecov Report

Merging #14 into master will decrease coverage by 38.03%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master      #14       +/-   ##
===========================================
- Coverage   97.76%   59.72%   -38.04%     
===========================================
  Files           1        2        +1     
  Lines         134      221       +87     
===========================================
+ Hits          131      132        +1     
- Misses          3       89       +86     

Impacted Files	Coverage Δ
src/LocalRegistry.jl	97.77% <ø> (+0.01%)	⬆️
src/merge.jl	0.00% <0.00%> (ø)

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[musm]

This would indeed be very helpful functionality. My use case:
Custom registry with packages copied over from the General registry + custom user packages. The ability to copy over packages from the registry would simplify setting up the custom registry.

[GunnarFarneback]

This is a bit of odd-ball functionality for LocalRegistry and nothing I have needed since I created the PR. I don't know how well it has aged but what it's really missing is tests.