fix InstallPackage for prescribed versions

[ThomasBreuer]

If an exact version number (one starting with =) is prescribed in InstallPackage, the exactness was ignored up to now. With this change, an available package version with larger version number is not regarded as matching the version condition in this case, and the return value is false.

This affects InstallPackageByName and InstallPackageByInfo. (The possibility to prescribe a version number in the latter function is in fact not documented.)

The problem had been observed already in issue #114. Its discussion makes clear that (currently) PackageManager cannot access arbitrary older versions of GAP packages. My understanding is that InstallPackage must return false if an exact version number is requested and if if this version cannot be made available.
Should the documentation be made clearer in this respect?

Sorry for the inconveniences, in particular for not reacting earlier.
The reason for the bug was a wrong usage of GAP's CompareVersionNumbers.
(All code involving this function could be simplified if this function would deal with the meaning of version numbers starting with =.)

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.42%. Comparing base (e4324af) to head (b147909).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/distro.gi	66.66%	2 Missing ⚠️
gap/packageinfo.gi	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   86.44%   86.42%   -0.03%     
==========================================
  Files          22       22              
  Lines         996     1009      +13     
==========================================
+ Hits          861      872      +11     
- Misses        135      137       +2     

Files with missing lines	Coverage Δ
gap/compile.gi	93.02% <100.00%> (-0.60%)	⬇️
gap/packageinfo.gi	91.78% <91.66%> (-0.87%)	⬇️
gap/distro.gi	81.72% <66.66%> (-0.15%)	⬇️

[mtorpey]

This is good – I'd forgotten about this weirdness with the = sign, and this change makes things much more sensible.

Thanks also for fixing the tests I broke yesterday!