Make briefcase dev update dependencies automatically

[mhsmith]

Same as #807, but for briefcase dev. It currently installs dependencies only when there's no .dist-info directory, which usually means it'll only do it on the first run.

[freakboy3742]

This is a good idea from a UX point of view; forgetting to run dev -d is a bit of a pain. However, the naïve approach of "just turn on -d for every run" isn't ideal either, because running pip install on a list of dependencies that hasn't changed takes a non-trivial amount of time. briefcase dev is a command where execution time matters, so we need to be careful that any mechanism to ensure dependencies are installed doesn't massively slow down the execution experience. As with #807, it might be possible to do something with a hash of pyproject.toml - if you modify pyproject.toml, the hash changes, and briefcase dev tries to install dependencies.

There's also some overlap with #431 - ensuring that you haven't installed a local package and need to add something to your pyproject.toml. However, off the top of my head, I can't think of an obvious hashing target to use for this.

[mhsmith]

Last week we talked about the idea of making Briefcase inspect the environment using something like importlib.metadata, and comparing it with the requirements to see whether it needs to re-run pip. But that wouldn't be so simple for requirements that are local directories, sdists, wheels, or Git URLs. So using a hash of the requirements list is probably a better idea.