Replace uses of `strip_extended_path_prefix` with `path::absolute`

[wmmc88]

This module is meant for private utils contained for use within this crate. It looks all that's used in PathExt and StripExtendedPathPrefixError. I think both those things are obsolete now that https://doc.rust-lang.org/std/path/fn.absolute.html is available on stable. I originally wrote that code before it was stabilized.

All uses of PathExt because of canonicalize should be changed to use path::absolute and other stdlib functions. The utils module should also be returned to being private.

This is a wdk-build release blocker.

Originally posted by @wmmc88 in #306 (comment)