PageSource should show local file

[frankluebeck]

So far, we assumed in PageSource that if FILENAME_FUNC returns a string not
starting with '/' that the pathname was relative to a GAP root. This is not
true if a function was read from a file relative to GAPs current directory.

Remark: It would be good to store a full path with function read from a
local file.

[codecov]

Codecov Report

Merging #2606 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2606      +/-   ##
==========================================
- Coverage    74.8%   74.79%   -0.01%     
==========================================
  Files         479      479              
  Lines      242243   242245       +2     
==========================================
- Hits       181198   181195       -3     
- Misses      61045    61050       +5

Impacted Files	Coverage Δ
lib/methwhy.g	49.58% <0%> (-0.42%)	⬇️
hpcgap/lib/hpc/stdtasks.g	63.42% <0%> (-0.77%)	⬇️

[ssiccha]

Sounds and looks sensible to me.