V2 cli: add external commands

[jeffa5]

trafficstars

Similarly to how cargo searches PATH for binaries named cargo-* and treats them as subcommands with args passed through it would be cool to have this ability in tectonic, searching for tectonic-*, to allow other tools to be created quickly to experiment on new ideas.

[pkgw]

Yup. I've got analogous code in Cranko that could be used as a template.

(which was itself pretty much directly lifted from Cargo)

[4e554c4c]

This issue should be closed, no?