In previous releases, it wasn't possible to declare a version catalog alias which was an
intermediate node, but also provided a value. For example, this was invalid (using TOML
but the API had the same restrictions):

   [libraries]
   top="org:foo:1.0"
   top.bottom="org:bar:1.0"

The reason is that we need the leaves to be providers and there's no custom implementation
of a provider for dependency notations (custom implementations are expensive to maintain
and should be avoided).

To workaround this problem, this commit introduces an interface, `ProviderConvertible`,
which is implemented by intermediate nodes which also provide a value. This interface
provides a single method, `asProvider()`, which is transparently called by Gradle when
such a notation is added as a dependency:

    implementation(libs.top) // libs.top implements ProviderConvertible

so this is equivalent to writing:

    implementation(libs.top.asProvider())

And accessing the leaf is possible as before using:

    implementation(libs.top.bottom)

Fixes #17170

This commit fixes a null pointer exception whenever a catalog references aliases
using different separators than what was expected. This was accidentally found when
fixing the nested alias structure, which in some cases may use the wrong alias because
it builds paths using `.` instead of what would be declared in a catalog.

Now it doesn't matter how the client asks for an alias/bundle/version: it is always
normalized when building the catalog or querying it.