Cross-project dependence

[guybedford]

Currently we don't support backtracking in targets or deps to reference paths outside of the folder with the chompfile.toml root. This makes it impossible to do mtime-based comparisons for operations outside of the chompfile therefore requiring cross-project tasks to be re-run everytime and instead require their own invalidations.

Just adding support for arbitrary backtracking in targets and dependencies might be a good start for cross-project validations.

Further support for building multi-chompfile graphs based on treating files as "belonging to" their parent chompfile might then further provide something like a monorepo support.

Worth fleshing this out further and carefully considering the consequences, but it could be worthwhile if there is interest.