Validate PackageInfo.g when loading packages

[fingolfin]

This helps package authors notice earlier if a package update they made failed.

One could also be more aggressive and validate every package record as we load it; but I have far too many packages which are not validating sitting around (e.g. undeposited ones from gap-packages)

[olexandr-konovalov]

Nice - from the packages redistributed with GAP, there is only one that triggers a warning:

Loading crime ... 
#E  component `Date' must be bound to a string of the form `dd/mm/yyyy'
#E Validation of package crime from /home/travis/build/gap-system/gap/pkg/crime failed 

because its date is 01/6/2011 and there were no new releases ever since.

[codecov]

Codecov Report

Merging #2726 into master will decrease coverage by 0.48%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2726      +/-   ##
==========================================
- Coverage   76.37%   75.88%   -0.49%     
==========================================
  Files         431      481      +50     
  Lines      220298   241315   +21017     
==========================================
+ Hits       168251   183123   +14872     
- Misses      52047    58192    +6145

Impacted Files	Coverage Δ
lib/package.gi	71.06% <66.66%> (+1%)	⬆️
lib/session.g	56.86% <0%> (-33.77%)	⬇️
src/gapstate.h	71.42% <0%> (-28.58%)	⬇️
lib/random.g	93.33% <0%> (-6.67%)	⬇️
src/calls.h	95.74% <0%> (-4.26%)	⬇️
src/objects.c	79.81% <0%> (-1.89%)	⬇️
src/iostream.c	63.49% <0%> (-1.49%)	⬇️
src/system.c	70.18% <0%> (-1.48%)	⬇️
src/gvars.c	82.6% <0%> (-1.24%)	⬇️
src/saveload.c	68.25% <0%> (-0.92%)	⬇️
... and 119 more

[olexandr-konovalov]

@fingolfin can you rebase, and I will be happy to approve this?

[fingolfin]

Rebased.