add install_src option and enhance buildcmd option in PythonPackage easyblock

[Flamefire]

In e.g. easybuilders/easybuild-easyconfigs#13622 we need to run Bazel to generate a whl file and then install that using pip. This is currently unsupported by the PythonPackage easyblock.

This PR allows this by setting:

use_pip = True
buildcmd = '%(python)s build/build.py`
install_src = 'dist/*.whl'

Also some minor moving around:

• Setting PYTHONNOUSERSITE a bit earlier
• Combining 2 very related run_cmd into one as external commands have a lot overhead

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS Amara-1.2.0.2-foss-2019a-Python-2.7.15.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
taurusi6606.taurus.hrsk.tu-dresden.de - Linux RHEL 7.9, x86_64, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (haswell), Python 2.7.5
See https://gist.github.com/02a8cf29f47c3f4e90a161f428c17cfc for a full test report.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS poetry-1.0.9-GCCcore-9.3.0-Python-3.8.2.eb
• SUCCESS Amara-1.2.0.2-foss-2019a-Python-2.7.15.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
taurusi6606.taurus.hrsk.tu-dresden.de - Linux RHEL 7.9, x86_64, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (haswell), Python 2.7.5
See https://gist.github.com/512908e5ebb263072d9d6ccbf9e17476 for a full test report.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS poetry-1.0.9-GCCcore-9.3.0-Python-3.8.2.eb
• SUCCESS Amara-1.2.0.2-foss-2019a-Python-2.7.15.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
taurusa5 - 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/c185f02d3a66c169a298e8e654c4f7f2 for a full test report.

[boegel]

@Flamefire Hmm, I ran into an easyconfig that's setting buildcmd that is failing with these changes, resulting in:

/bin/bash: build_ext: command not found

(see test report for libgpuarray)

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• FAIL (build issue) libgpuarray-0.7.6-fosscuda-2020b.eb (partial log available at https://gist.github.com/0da30337793bf65cc08baf556b53afbf)

Build succeeded for 0 out of 1 (1 easyconfigs in total)
node3302.joltik.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6242 CPU @ 2.80GHz (cascadelake), Python 3.6.8
See https://gist.github.com/9b4d4b36a69b4938630c2d44bf50e979 for a full test report.

[Flamefire]

@boegel That is actually a good thing. The EC sets buildcmd but uses use_pip=True which means it was silently unused.
We should check and probably fix that EC

[Flamefire]

As far as I can tell using pip works fine, so the buildcmd should be removed: https://github.com/Theano/libgpuarray/blob/9cec61435c11cf2ffcf7c480c2d524772585c47f/doc/installation.rst

[boegel]

@boegel That is actually a good thing. The EC sets buildcmd but uses use_pip=True which means it was silently unused.
We should check and probably fix that EC

Ah, that explains it, I was having trouble understand why the changes in this PR could cause trouble like that 😅

I'll look into fixing the libgpuarray easyconfigs by stripping out the unused buildcmd

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Meson-0.58.2-GCCcore-11.2.0.eb
• SUCCESS archspec-0.1.2-GCCcore-10.3.0.eb
• SUCCESS h5py-3.2.1-foss-2021a.eb
• SUCCESS CellRank-1.4.0-foss-2021a.eb
• SUCCESS matplotlib-3.4.2-foss-2021a.eb
• SUCCESS QUAST-5.0.2-foss-2020a-Python-3.8.2.eb
• SUCCESS IPython-7.25.0-GCCcore-10.3.0.eb
• SUCCESS Python-3.8.6-GCCcore-10.2.0.eb
• SUCCESS Python-2.7.18-GCCcore-9.3.0.eb
• SUCCESS sklearn-som-1.1.0-foss-2020b.eb
• SUCCESS PyQt5-5.15.1-GCCcore-10.2.0.eb

Build succeeded for 11 out of 11 (11 easyconfigs in total)
node3112.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/1091fbd99826d9fc17dde88f60862b17 for a full test report.