Unfortunately, the diagnotic machinery does not cope well with an empty
span which can happen if the crate is empty, in which case we merely set
a spanless note.
This allows lints and other diagnostics to refer to items
by a unique ID instead of relying on whacky path
resolution schemes that may break when items are
relocated.
Normally `#![feature(...)]` shouldn't change behavior, but custom attributes in particular are in the process of retirement, and we should not produce a message telling to enable them.
It also helps with unifying diagnostics for unresolved macros.
This way we are processing all of them in a single point, rather than separately for each syntax extension kind.
Also, the standard expected/found wording is used.
It either returns the indeterminacy error, or valid (but perhaps dummy) `SyntaxExtension`.
With this change enum `Determinacy` is no longer used in libsyntax and can be moved to resolve.
The regressions in diagnosics are fixed in the next commits.
