Make use of the newly introduced --prefix option to the pythonpath
command, and generate what's subseqently used to fill in mypy_path in
pyproject.toml.
By decoupling it from PYTHONPATH, this commit makes the creation of
mypy_path less involved and easier to understand. It also obviates
the need replace the relatively heavy ldlibpath.mk by the relatively
lightweight projects.mk, thereby enhancing performance.
Signed-off-by: Jan Lindemann <jan@janware.com>