change 'eb' command to import easybuild to check if it's working

[bartoldeman]

This is a much cheaper test than import easybuild.main and still
solves the same issue as #3610.

This reduces the wall time for eb --version from 1.35s to 0.73s
in our configuration.

[bartoldeman]

See also #3180

[casparvl]

Looks good to me. I've installed from the feature branch, and tested timings. For me it was about 25% faster to run eb --version. I've also tested a basic installation (which worked fine) just as a sanity check, even though this PR shouldn't change anything that could affect installations - only the check for which python to use was touched.

To me, the change looks sane and should still return the correct Python, namely the one used to install EasyBuild.

Requested changes were implemented

[boegel]

This is actually not equivalent at all, since if somehow only the eb command and the easybuild-easyblocks package are available, import easybuild will also work, but easybuild.main (or anything else provided by framework) may not be, so I'm not sure this change is a good idea...

[boegel]

An alternative could be to test the import of easybuild.framework, which is very light weight as well (doesn't import other things from easybuild namespace), and that's definitely a better check to verify that EasyBuild framework is available...

Follow-up PR at #3998