Error if globs don't match any input files

[aomarks]

We should error if a glob in the files array doesn't match any files.

Note we have to do this at Execution time, not Analysis time, because the input files might not exist before dependencies execute.

Also note currently we can only do this crudely: "are any files matched by these input globs" because we don't have insight into which specific globs matched, because we delegate globbing to fast-glob.

However, a new replacement globbing library is in progress, which will allow tracing every entry to the original glob that produced it.

[cefn]

This would have unfortunate consequences for optional files. For example, I currently have a rule for my Vitest target, which includes a match for the optional vite.config.ts and vitest.config.ts files even though they are not there.

That way, if I add one, the specialisation is respected as part of the test definition, but it's legitimate for it to be missing (which causes the suite to run with Vitest fallbacks).