File provider key that represents a root file provider.

Root file providers have no indirection and will access the underlying filesystem directly.

There is an UUID generated automatically for every local root file provider which they are uniquely identified by. This UUID is usually generated one time for every machine that ever runs the build system. (It is generated the first time LocalFileProvider is used, and the provider key UUID is stored in the default storage directory of the build system.)

Note that it is not required that all root file provider implementations are backed by LocalFileProvider.