Set package search paths independently from GAP roots

[fingolfin]

This is work-in-progress code which I started in February, but then got sidetracked, and never completed.

Before I invest more effort into it, I thought I should ask what people think about this...

This PR is about changing GAP to tracker "package search directories" separately from "GAP roots"

The idea is that I would like to be able to tell GAP to load packages from a specific dir, and not just from a "foo/pkg/my-actual-package", where "foo" must be a GAP root, and "pkg" is fixed (GAP insists on it). Instead, I'd like to be able to just point GAP at a directory "foo", and it automaticaly picks up any packages in there -- regardless of whether "foo" itself is the directory of a package, or "foo" contains packages (such as "foo/cvec", "foo/orb", ...).

Of course one can still specify GAP roots; internally, when you specify a GAP root "foo", then GAP simply adds "foo/pkg" to the list of package search directories. But in addition, users should be able to specify package search directories which are not within a GAP root. Either by calling the GAP function ExtendPackageDirectories(), or by specifying a command line flag (the latter has not yet been implemented).

Of course if anybody is interested in this, feel free to help work on it... or just give feedback... :)

[codecov-io]

Codecov Report

Merging #933 into master will decrease coverage by <.01%.
The diff coverage is 34.61%.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
- Coverage    74.6%    74.6%   -0.01%     
==========================================
  Files         481      481              
  Lines      242402   242427      +25     
==========================================
+ Hits       180851   180858       +7     
- Misses      61551    61569      +18

Impacted Files	Coverage Δ
lib/system.g	67.28% <100%> (+0.15%)	⬆️
lib/package.gi	69.38% <32%> (-0.44%)	⬇️
hpcgap/lib/hpc/stdtasks.g	63.68% <0%> (-0.26%)	⬇️

[olexandr-konovalov]

I think this may be useful - for example, this would make life easier when one runs GAP docker container and would like to load GAP package from their local filesystem. I suggest to review this in order to include in GAP 4.9.

[fingolfin]

To finish this, there should also be a command line option similar to -l but for package dirs instead of GAP roots. See also issue #12

[olexandr-konovalov]

@fingolfin this seems not be ready for GAP 4.9.0 - let's postpone to some future milestone.

[fingolfin]

@alex-konovalov well, you added that milestone, I never had that in mind :-). Removed it.

[olexandr-konovalov]

@fingolfin ok. That was just an "add that milestone to remind us to review the status of this PR before making GAP 4.9 beta" intention.

[fingolfin]

Closing in favor of PR #5873