Creation of multiple `InProject` resolvers on the same project shouldn't be allowed

This will just lead to confusion. Once the project gets crated a default `InProject` resolver gets created as well. That should be enough and in practice no other `InProject` should be created.

Allowing updates on it seems fine to me (even though the people doing that should really know what they are doing, since they can give more priority to a `CrossProject` resolver than an `InProject` which seems very weird, but possible and even useful under certain situations).

No files, tests, or entry points are identified in the issue. Start by tracing project creation and InProject resolver creation, then locate the validation path for resolver creation. Done means a project cannot create a second InProject resolver while updates to the default resolver remain allowed.

Written by the indexing model from the issue text.