Invoke a recipe from another file as a dependency

[bhollis]

As we expand our usage of just, we'd like to be able to move recipes between modules in our repo. I know we can import rules between files, but it'd be great if we could simply invoke rules in other files. e.g.:

./justfile:

foo: bar/baz
  echo "this happens after bar's baz recipe"

./bar/justfile:

baz:
  echo "it's baz time!"

I can run just bar/baz at the CLI and it'll work, but declaring bar/baz as a dependency of foo doesn't work the same way:

$ just bar/baz
echo "it's baz time!"
it's baz time!
$ just foo
error: Expected '&&', comment, end of file, end of line, identifier, or '(', but found '/'
 ——▶ justfile:1:9
  │
1 │ foo: bar/baz
  │         ^

[casey]

This is desired, but not yet implemented. See #929.