Invalidate cache when prjfmt.toml is modified

[basile-henry]

At the moment, when the state of the repo is cached, modifications to the prjfmt.toml file are not used until the repo is modified because prjfmt assumes everything is already formatted.

I assume that if a formatter is updated/version changed, then a similar issue where prjfmt will assume it doesn't need to run arises.

This logic of cache invalidation can probably become complicated, maybe we could just add a command to the CLI to let the user invalidate the cache manually: prjfmt --clear-cache?

[zimbatm]

I think this can be fixed with the following changes:

• expand the path of the formatter binary to get an absolute value. Record that into the manifest.
• record the mtime of the formatted binary
• when checking if a file has "changed", also compare the command that has been used.

That should capture 99% of the cases.

There is still the issue that the formatters themselves might have a config file that gets changed so adding the --clear-cache option will be necessary.
The cache should compare (cmd, mtime) instead of just the time. That way if the command changes or a new version of the software is installed, it will re-run the formatter.