refactor(boot): remove -map (#12452)

This flag does not reflect how dune's regular rules work. Let's get rid
of it and resolve module names using aliaes ourselves.

This is needed for `(include_subdirs qualified)` as well. Not all alias
files can be visible in this mode.

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg