Add 'ignore-non-existent-contents' to tell the VFS whether an invalid path
obtained via 'external-contents' should cause iteration on the VFS to stop.
If 'true', the VFS should ignore the entry and continue with the next. Allows
YAML files to be shared across multiple compiler invocations regardless of
prior existent paths in 'external-contents'. This global value is overridable
on a per-file basis.
This only adds the parsing part. Logic to actually use it will come next.