(PUP-2654) Capitalize each segment of a resource type "label"

When a Resource type is stringified it selects the short notation
(e.g. Fee::Foo) over the longer (e.g. Resource[Fee::Foo]), and
when doing so it should output each segment of the FQN with a
capital letter. The current implementation only capitalizes
the initial letter (e.g. Fee::foo::fum).

This fixes the implementation of
TypeCalculator.string_PResourceExpression to capitalize all segments.
(maint) Extract constant for name separator

The name separator is used a few different times in the file. This
extracts it out and freezes the value, which has the nice benefit of a
small speed boost. Unfortunately this isn't a big win for the system
since this code path is only really used for error conditions.
Merge branch 'pr/2810'

* pr/2810:
  (maint) Extract constant for name separator
  (PUP-2654) Capitalize each segment of a resource type "label"