`rdmd -deps=foo.deps foo.d` caching misses dependency on foo.deps

**Transferred from**: https://issues.dlang.org/show_bug.cgi?id=17071

User @timotheecour reported (2017-Jan-08 02:57:48):
> ```console
> $ rdmd -deps=foo.deps -c foo.d
> #creates foo.deps foo.o
> $ rm foo.deps
>
> $ rdmd -deps=foo.deps -c foo.d
> # BUG: foo.deps not recreated
>
> $ rdmd --force -deps=foo.deps -c foo.d
> #ok
> ```