Better testing for monorepo projects in isolation

### Description

https://github.com/alleyinteractive/mantle-framework/pull/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: https://github.com/alleyinteractive/wp-modified-date-control/pull/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`.