This re-enables documentation lost during refactoring work, under the
target _NIOFileSystem
resolves#3474
### Motivation:
During the refactoring of NIOFileSystem and recent Swift updates, the
mechanism to "shadow" symbols using `@_exported import` has stopped
working for documentation and imports for symbols, which means that
_NIOFileSystem is the target that needs to host the documentation for
this (for now)
### Modifications:
Moves DocC catalog into _NIOFileSystem target, and updates
disambiguation hashes on overloaded symbols in order to verify no
warnings are presented while generating documentation.
Updates .spi.yml to present _NIOFileSystem instead of NIOFileSystem
### Result:
Previous documentation should be available again, although at a slightly
different URI structure within Swift Package Index.
Co-authored-by: Cory Benfield <lukasa@apple.com>
Joseph Heck