This commit updates the debuginfo that is encoded in all of our released
artifacts by default. Currently it has paths like `/checkout/src/...` but these
are a little inconsistent and have changed over time. This commit instead
attempts to actually define the file paths in our debuginfo to be consistent
between releases.

All debuginfo paths are now intended to be `/rustc/$sha` where `$sha` is the git
sha of the released compiler. Sub-paths are all paths into the git repo at that
`$sha`.