This adds dep-info tracking for non-path dependencies. This avoids tracking
package-relative paths (like source files) in non-path dependencies, since we
assume they are static. It also adds an mtime cache to improve performance since
when rustc starts tracking sysroot files (in the future), it can cause a large
number of stat calls.

Instead of treating Windows differently, this just always uses canonical paths
on all platforms.  This fixes a problem where symlinks were not treated
correctly on all platforms.

Switching rm_rf to the remove_dir_all crate because deleting symbolic links on
Windows is difficult.