Make mmc --make account for source files disappearing.

mmc --make prints a grave error message when the source file of a module
can no longer be found:

** Module `foo' is imported or included by module `bar'.
** dependencies for `Mercury/int3s/foo.int3' do not exist: foo.m
** This indicates a bug in `mmc --make'.

compiler/make.module_dep_file.m:
	Check that the source file of a local `.module_dep' file still
	exists.  If it doesn't, delete the `.module_dep' file and ignore
	the information it contains.