Get rid of `load`

Replace `load` with a manual `read` of the Carton file, following by a
manual evaluation of all directives on top of `pcase`.

This is more secure than `load`, and has a cleaner namespace, because
all the unprefixed functions (e.g. `depends-on`, `development`, etc.)
are gone now.

Also make the error message adhere to the conventions for error
messages.