Change Cargo's own dep-file format

Change Cargo's own dep-file format

This commit alters the format of the dependency info that Cargo keeps track of
for each crate. In order to be more resilient against directory renames and such
Cargo will now postprocess the compiler's dep-info output and serialize into its
own format. This format is intended to primarily list relative paths *to the
root of the relevant package* rather than absolute or relative to some other
location. If paths aren't actually relative to the package root they're still
stored as absolute, but there's not much we can do about that!