Allow changing the name or namespace of ThisAssembly class

We encountered the following scenario where `ThisAssembly caused a build break.

1. Project A has ThisAssembly generated and also [InternalsVisibleTo] for project B
2. Project B also has ThisAssembly. When it tries to use it, build fails because of ambiguous ThisAssembly class.

The solution to this issue may be either naming ThisAssembly class differently, or placing it inside a namespace (root namespace of the project, perhaps). For backward compatibility, this should be opt-in.

[alexrp]

@AArnott Given the above, can we reopen this? (I’ll try to find time to do a PR for this eventually, but in the meantime, someone might come across this and feel like contributing the enhancement.)

[AArnott]

I don’t think we’d add a boolean switch to forcibly link the namespace of ThisAssembly to RootNamespace. Rather, if we made the namespace flexible I’d say just take a string switch and let it be anything (including $(RootNamespace)).