## Why
`PathUri::join` should not depend on the app-server compatibility
wrapper `LegacyAppPathString` to parse native paths. Native path parsing
belongs to the URI abstraction that it constructs.
## What
Move platform-independent native path parsing into the root `PathUri`
module. `PathUri::join` and `LegacyAppPathString` now share the
crate-private `PathUri::from_absolute_native_path` constructor.