Grails 4.0.4: Namespaced controllers being matched without providing namespace to the mapping

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### Task List

- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

1. Perform run-app on the linked repository
2. Visit the path http://localhost:8080/namespaced-should-not-work-but-works

### Expected Behaviour

The documentation (https://docs.grails.org/4.0.4/guide/theWebLayer.html#namespacedControllers) says:

> When resolving a URL mapping (forward or reverse) to a namespaced controller, a mapping **will only match** if the namespace has been provided. If the application provides several controllers with the same name in different packages, at most 1 of them may be defined without a namespace property. If there are multiple controllers with the same name that do not define a namespace property, the framework will not know how to distinguish between them for forward or reverse mapping resolutions.

According to the doc, a mapping for a namespaced controller will **only** match if the namespace has been provided, so therefore /namespaced-should-not-work-but-works should not work because I did not provide a namespace to the mapping but it does.

Expected behavior of the example app:
|Path|Expected Result|
|---|---|
|/namespaced-should-not-work-but-works|**Should not work**|
|/namespaced|Should work|
|/namespaced-should-not-work|Should not work|
|/ordinary|Should work|
|/ordinary-should-not-work|Should not work|

I am relying on this behavior to avoid accidentally mismatching a controller.

### Actual Behaviour

However, when I added a mapping for a namespaced controller **without** providing the namespace, it still worked. The "/namespaced-should-not-work-but-works" URL mapping defined in the example app still worked.

Actual behavior of the example app:
|Path|Actual Result|
|---|---|
|/namespaced-should-not-work-but-works|**Works**|
|/namespaced|Works|
|/namespaced-should-not-work|Does not work|
|/ordinary|Works|
|/ordinary-should-not-work|Does not work|

### Environment Information

- **Operating System**: macOS Catalina 10.15.4
- **Grails Version:** 4.0.4
- **JDK Version:** JDK 1.8 Update 151
- **Container Version (If Applicable):** N/A

### Example Application

- https://github.com/rocketmiles/grails-test-namespaced-controller

Start with the linked example application and reproduce the listed namespaced and ordinary URL paths. Trace the URL-mapping and namespaced-controller resolution entry points, then add coverage showing that a mapping without a namespace does not match a namespaced controller while the documented paths continue to work.

Written by the indexing model from the issue text.