mk: Use rwildcard to calculate dependent files

The previous dependency calculation was based on an arbitrary set of asterisks
at an arbitrary depth, but using the recursive version should be much more
robust in figuring out what's dependent.

Closes #13118
auto merge of #13142 : alexcrichton/rust/issue-13118, r=brson

The previous dependency calculation was based on an arbitrary set of asterisks
at an arbitrary depth, but using the recursive version should be much more
robust in figuring out what's dependent.