put fake 'vsc' namespace in place that produces an error on import

[boegel]

Since we ingested vsc-base & vsc-install into the EasyBuild framework (cfr. #2708) we should actively prevent imports from the vsc.* namespace, since it can cause problems (see for example #2745).

This injects a fake vsc namespace early on, that catches any import vsc.* or from vsc.* import ... statements, and exits with a clear error, for example:

$ eb GCCcore-6.3.0.eb
== temporary log file in case of crash /tmp/eb-c1eTh9/easybuild-yR5zsp.log

ERROR: Detected import from 'vsc' namespace in /Users/kehoste/work/easybuild-easyblocks/easybuild/easyblocks/g/gcc.py (line 42)
vsc-base & vsc-install were ingested into the EasyBuild framework in EasyBuild v4.0
The functionality you need may be available in the 'easybuild.base.*' namespace.