Better testing for monorepo projects in isolation

[srtfisher]

Description

#816 arose because it referenced an interface from the types package within support. On a normal project, this is usually fine. But a monorepo like mantle-framework this resulted in an undefined class error:

Class "Mantle\Types\Validator" not found

Example project that uses support and testkit but did not import the newer types package and failed because of this: alleyinteractive/wp-modified-date-control#99

We need to figure out how we can test our subprojects in better isolation from one another. Perhaps there is a CI step that can check for cross-project references without being explicitly declared in the subproject's composer.json.

[srtfisher]

Another example that would have been great to catch: #825