Add GAP.Packages.build(name)

[lgoettgens]

Some progress towards #1065.

As pointed out in #1065 (comment), this function may need to copy a package from an artifact directory to the user pkgdir to have it in a location that is both writable and PackageManager:CompilePackage does not complain about.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 74.68%. Comparing base (8f4a1a1) to head (b1b36f6).

Files with missing lines	Patch %	Lines
src/packages.jl	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1066      +/-   ##
==========================================
- Coverage   74.97%   74.68%   -0.30%     
==========================================
  Files          55       55              
  Lines        4560     4550      -10     
==========================================
- Hits         3419     3398      -21     
- Misses       1141     1152      +11     

Files with missing lines	Coverage Δ
src/packages.jl	60.46% <0.00%> (-12.44%)	⬇️

... and 8 files with indirect coverage changes

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