Don't unpack whl files by default in generic PythonPackage easyblock

[Flamefire]

Especially with the introduction of easybuilders/easybuild-framework#3618 Wheels may become more common as (THE) valid installation approach for Python packages via pip. Having to specify unpack_sources = False every time is annoying and feels like something is off.
This basically adds first-class support whl-files by NOT unpacking them by default.

This could be considered a breaking change for cases where whl files were used and they should be unpacked without explicitly specifying unpack_sources = True (as that was the default), but I don't think it was even possible to do that meaningfully, so I'd say the valid cases that this breaks are unicorn rare.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS pkgconfig-1.5.1-GCCcore-8.2.0-python.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
taurusa4 - Linux centos linux 7.7.1908, x86_64, Intel(R) Xeon(R) CPU E5-2603 v4 @ 1.70GHz (broadwell), Python 2.7.5
See https://gist.github.com/6e391ca923ed2e51057ea6df0fbbae63 for a full test report.

[hound]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 5, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[Flamefire]

This is a test to show a failing report only!

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• FAIL (build issue) pkgconfig-1.5.1-GCCcore-8.2.0-python.eb (partial log available at https://gist.github.com/cf88a695c759abe3149dd33dfa3a073c)

Build succeeded for 0 out of 1 (1 easyconfigs in total)
taurusa4 - Linux centos linux 7.7.1908, x86_64, Intel(R) Xeon(R) CPU E5-2603 v4 @ 1.70GHz (broadwell), Python 2.7.5
See https://gist.github.com/859650686eab87d18c197ce207283f32 for a full test report.

[Flamefire]

Rebased to resolve the conflict

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Ray-project-1.0.1-fosscuda-2019b-Python-3.7.4.eb
• SUCCESS Python-3.8.6-GCCcore-10.2.0.eb
• SUCCESS SciPy-bundle-2020.11-foss-2020b.eb
• SUCCESS matplotlib-3.3.3-foss-2020b.eb

Build succeeded for 4 out of 4 (4 easyconfigs in total)
node2660.swalot.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 3.6.8
See https://gist.github.com/89dea4a9bb5f0a1ab6d632f47ca8950f for a full test report.