Rationalize how to compare namespaces across razor/roslyn

[ryzngard]

In #11146 some tests were adding in cases for @using Goo = Bar and it's noted that there are two ways to get a namespace off a directive: a Namespace and a ParsedNamespace. Unfortunately, neither string accurately represents Goo = Bar in this case. The former is Goo = Bar and the latter is Goo=Bar.

This will need to correctly be handled for cases where these namespaces to be removed. I don't see this as too much of a blocking issue as of yet so I'm filing a follow up issue.