> I wonder if for command runner unnecessarily re-running dependencies is really a big deal.

I've used in in the past with python/django roughly like so (untested from memory, there may be a "last modified" bug in here that still makes something run unnecessarily):

  .PHONY: runserver

  environ:
    python -m venv $@

  environ/lib: requirements | environ
    . environ/bin/activate && pip install -r $<
    touch $@

  runserver: | environ/lib
    . environ/bin/activate && python manage.py runserver [foo]

Setting up these prerequisites takes a while and doing every time you start the dev server would be a pain, but not doing it and forgetting when requirements were updated is also a pain. This would handle both.