Improve Python package version check and add unversioned_packages EC param

[Flamefire]

This fixes a format argument confusion in the check for a 0.0.0 version and enhances the error message.
As an escape hatch I added an EC param unversioned_packages which contains Python package names which should not trigger a failure on a 0.0.0 version as a last resort, e.g. as of recently in the opencarp package: easybuilders/easybuild-easyconfigs#12462

As those checks can be annoying as they run after a potentially long installation I bundled all error messages together.

As it is impossible to detect the package name used by pip list automatically, the unversioned_packages must be set even in ECs using other ECs which have such packages. I expect that to be very rare though.
As an explanation: EC "python-protobuf` has a modulename of "google.protobuf" and a pip name of "protobuf". The Keras-* packages are likely similar. Reason is that the name used by pip is in the meta data of the package and could theoretically be anything, IIUC

[boegel]

tiny typo to fix...

[boegel]

lgtm

Tested with both an easyconfig file that now fails due to a 0.0.0 version being installed (carputils-20200915-foss-2020a-Python-3.8.2.eb), and verifying that unversioned_packages works as expected, and with PythonPackage easyconfigs that are not affected.