PythonPackage is not compatible with --sanity-check-only

[boegel]

Using --sanity-check-only on something that was installed with the PythonPackage (or PythonBundle, or a derivative) easyblock fails with:

== FAILED: Installation ended unsuccessfully (build directory: /tmp/easybuild_build/pygmo/2.16.1/foss-2020b): build failed (first 300 chars): cmd "pip --version" exited with exit code 127 and output:
/bin/bash: pip: command not found
 (took 0 secs)

The problem is that the module (which is already installed) only gets loaded later, when the parent sanity_check_step is being called.

[mboisson]

This caused a regression with multi_deps. See the linked PR to fix it.