Previously when looking up cached provider input, the Input was taken in
its entirety, and only provider configuration fields that weren't in the
saved input were added. This would cause providers in modules to use the
entire configuration from parent modules, even if they themselves had
entirely different configs.
Note: this is only marginally beter than the old behavior. It may be
slightly more correct, but stil can't account for the user's intent, and
may be adding configured values from one provider into another.
Change the PathCacheKey to just join the path on a non-path character
(|), which makes for easier debugging.
