Add support for aliased references

[kzu]

Whenever type name collisions exist, .NET offers aliases to disambiguate.
For example, you can add the Aliases metadata to a project reference as follows:

    <ProjectReference Include="..\Library1\Library1.csproj" Aliases="Library1" />
    <ProjectReference Include="..\Library2\Library2.csproj" Aliases="Library2" />

Then code must use:

extern alias Library1;
extern alias Library2;

To reference types in one or the other, even if the types have the exact same full name,
the compiler will be able to disambiguate.

Currently, we do not even process references that have aliases, meaning no services
will be registered at all. We should do so, and emit code that can compile by also
including the extern alias declarations as needed.