Make `raco setup` propagate the original value of
`use-compiled-file-paths` in case it's reset to avoid loading bytecode
files. Then, `--clean` mode can remove bytecode relevant to that
setting, instead of always clearing "compiled" subdirectories.
There's no external way to initialize `use-compiled-file-paths` right
now, other than forcing it to `null` with the `-c` flag at the
`racket` level, but the current "racket7" implementation uses
different `use-compiled-file-paths` settings for different build
modes, and it seems to make sense in general.
Also, make `--clean` sensitive to `-D` and `-d`, so that it's easy to
clean just bytecode.
`cext-lib` contains much of the contents of `dynext`, which
is no longer very widely used.
Also moved the implementation of the `mzc` executable
to a more appropriate package.
Also, used `lazy-require` consistently for dynamically
loading implementations.
