Fix inference for compact namespace declarations

[vinistock]

Motivation

Closes #2568

We weren't splitting the name parts from the nesting, which caused us to produce the incorrect type name Admin::User::<Class:Admin::User> as opposed to the correct singleton name Admin::User::<Class:User>.

That caused inference for self when invoking singleton methods on compact namespaces to be incorrect, which then makes completion, definition, hover and signature help not work.

Implementation

We just need to ensure that we're working with the individual name parts, so I just flat mapped the split.

Automated Tests

Added a few tests to ensure we don't regress.