Currently, poetry install will clone, build and install every git
dependency when it's not present in the environment. This is OK for
developer's machines, but not OK for CI - there environment is always
fresh, and installing git dependencies takes significant time on each CI
run, especially if the dependency has C extensions that need to be
built.

This commit builds a wheel for every git dependency that has precise
reference hash in lock file and is not required to be in editable mode,
stores that wheel in a cache dir and will install from it instead of
cloning the repository again.