Let skip_install override usedevelop instead of the other way around

[obestwalter]

skip_install and usedevelop can be used together in the same environment but the behaviour is not documented. If both flags are True the package will be installed although skip_install is requested (see also discussion here).

The behaviour should be changed that skip_install overrides usedevelop as usedevelop is an option to choose the installation type if the package is installed and skip_install determines if it should be installed at all.

This is a somewhat breaking change of undocumented behaviour but I think this should be done to make the behaviour intuitively correct (but it should also be documented).

It should be also stated prominently in the changelog that this changed and how it can be fixed though. The documentation should be updated mention the behaviour when these two flags are used together.

[obestwalter]

docs still needed.