Review `racket-mode-start-faster` in light of multiple back ends

[greghendershott]

When working on the branch for multiple back ends, including remote, we added code to possibly copy Racket Mode's back end source code (the *.rkt files) to the remote host if outdated, where "outdated" is determined using a digest of the source file contents.

However:

• racket-mode-start-faster has not (yet) been updated to do anything for remote back ends.

• Even for local back ends, we could probably use a digest to do a raco make automatically as/when needed. In other words there needn't be any racket-mode-start-faster command for the user to run manually.

• Furthermore -- and this is probably a separate issue and to-do -- aybe an info.rkt should define a package. This wouldn't be published on the Racket package catalog; the delivery mechanism for Racket Mode would still be an Emacs package that happens to contain Racket source (which people get from MELPA or use via straight.el or whatever). But it could mean that we can do raco pkg install --link. Let it do the raco make. Furthermore this could clarify the dependencies situation for users.