Allow non-types dependencies (#5769)

Remove the check in check_consistency that ensures that only other
type packages from typeshed are being depended on. Instead, add an
explanation to CONTRIBUTING that spells out the requirements. This
adds a higher, but reasonable burden on maintainers to check the
dependencies manually.

Part of #5768

CONTRIBUTING.md

@@ -94,7 +94,10 @@ supported:
   unknown and needs to be updated.
 * `python2` (default: `false`): If set to `true`, the top-level stubs
   support both Python 2 and Python 3.
-* `requires` (optional): A list of other stub packages that this package uses.
+* `requires` (optional): A list of other stub packages or packages with type
+  information that are imported by the stubs in this package. Only packages
+  generated by typeshed or required by the upstream package are allowed to
+  be listed here, for security reasons.
 * `extra_description` (optional): Can be used to add a custom description to
   the package's long description. It should be a multi-line string in
   Markdown format.