Use a single requirements file for all sessions

[cjolowicz]

We don't seem to need a per-session requirements file anymore. All sessions just need the full dependencies from the lock file in constraints form.

This came up a few times before:

• Installation of sdist is (silently) failing #249 (comment)
• Crash when using PassthroughEnv #347 (comment)

[cjolowicz]

One thing to keep in mind: Currently, requirements files only live as long as their environment. If it's placed outside of environments, there is no mechanism to clean up a broken requirements file. In other words, the caching will need to be more robust. For example, should we remove the file if it cannot be processed (or move it to $TMPDIR for post-mortem inspection)?

Another point, which is maybe quite specific to nox-poetry's own test suite: If poetry itself is installed inside the Nox environment, it can interfere with the exporting, with different versions of Poetry attempting to export from the lockfile. This should probably be a different issue.

[cjolowicz]

An implementation of this feature could use nox.Session.cache_dir introduced in Nox 2021.10.1.