Enhance sitecustomize.py script installed by Python easyblock to fix support for virtualenv

[Flamefire]

(created using eb --new-pr)

virtualenv seems to bundle a custom site.py which in versions prior to 20.something does not have site.getsitepackages(), see e.g. pypa/virtualenv#737

The error in that case is AttributeError: module 'site' has no attribute 'getsitepackages'

The workaround is a bit hacky but seems to work across various versions of Python/virtualenv.

Some problems considered/solved:

• base_prefix is supposed to contain the python prefix, but does not in early virtualenv versions, it instead contains the virtualenv prefix
• real_prefix was used instead but later removed
• Especially Python2 adds more than 1 folder to the getsitepackages
• Not all such entries end in site-packages

Hence the very conservative approach of trying getting the first of all possible prefixes and moving all paths starting with that after the EBPYTHONPREFIXES

I also added a sanity check to check the base case, i.e. site-customize does work at all and adds the new site-packages path before the core site-packages path. (conservative check, we might want to limit to moving paths only ending with site-packages)

Testing that this works with virtualenv is hard, as we'd actually need to install a virtualenv. Not sure if we really want to do that.

The script I used to verify the new site-customize.py: checkPythonModule.sh

[boegel]

@Flamefire Can you edit the PR description to mention the error messages that pop up unless this is included?

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Python-2.7.15-foss-2018b.eb
• SUCCESS Python-3.6.4-foss-2018a.eb
• SUCCESS Python-3.8.6-GCCcore-10.2.0.eb
• SUCCESS Python-3.9.5-GCCcore-10.3.0.eb

Build succeeded for 4 out of 4 (4 easyconfigs in total)
node3102.skitty.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, Python 3.6.8
See https://gist.github.com/a96c0da98ec645218d49263c1390ee2d for a full test report.

[boegel]

lgtm