[WIP] Perform pip -e install in configure step already.

Related to #279 (closed)

I propose to call pip install -e for a DUNE python module in the configuration step already, instead when building all. The editable install would be disabled when disabling the venv setup.

There is then only the target metadata_${envtargetname} remaining in all. Maybe this could also be removed when not using the venv. No target related to the venv setup is remaining in all any more.

@tkoch With this you would have no pip install -e and no metadata target any more when calling make all., and even no one left when disabling the venv. What do you think?

Current Status: We will incorporate these changes into the changes of !1148 (merged).

TODO:

• Replace the , separator in cmake_flags with something different by <SEP>.
• Incorporate the changes into !1148 (merged).