fix extension filter for Perl modules (WIP)

[boegel]

Current approach of checking whether Perl modules are installed fails to catch missing dependencies, cfr.:

$ perldoc -lm 'GD::Graph'
/prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/site_perl/5.26.1/GD/Graph.pm
$ echo $?
0

vs

$ perl -e 'require GD::Graph'
Can't locate GD.pm in @INC (you may need to install the GD module) (@INC contains: /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/site_perl/5.26.1/x86_64-linux-thread-multi /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/site_perl/5.26.1 /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/5.26.1/x86_64-linux-thread-multi /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/5.26.1) at /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/site_perl/5.26.1/GD/Graph.pm line 38.
BEGIN failed--compilation aborted at /prefix/software/Perl/5.26.1-GCCcore-6.4.0/lib/perl5/site_perl/5.26.1/GD/Graph.pm line 38.
Compilation failed in require at -e line 1.
$ echo $?
2

I made this WIP for now because fixing this impacts all Perl easyconfigs, and maybe other easyconfigs in which Perl extensions are involved, since the extension filter is also used during the sanity check...

Accompanying fixes will be required in these easyconfigs to avoid installations suddenly failing to complete.

[damianam]

@boegel ping? How are the fixes in the easyconfigs repo coming along? I see just 2 references here, fixing 4 files.

[boegel]

@damianam To be honest, I'm not sure... I would have to take a closer look, since this also affects the PerlModule easyblock.

[branfosj]

closing - we'll make this change with #2699