resolve: Make proc macro stubs less stubby

resolve: Make proc macro stubs less stubby

Create real working and registered (even if dummy) `SyntaxExtension`s for them.
This improves error recovery and allows to avoid all special cases for proc macro stubs (except for the error on use, of course).

The introduced dummy `SyntaxExtension`s can be used for any other inappropriately resolved macros as well.