Substitue "/" with "-" in the MODULES-IN phony goal names.

This avoids Ninja's path canonicalization on those goal names.
Without this, Ninja complains about duplicate rules for
MODULES-IN/a/b/.. and MODULES-IN/a.

Bug: 24309760
Change-Id: Id3aa9da5d3887a2cd1426cbb5fdf35659c65805e