Stacking attribute-based source generators via dependency graph

[alrz]

(This relates to #57239, dotnet/runtime#108317).

Since source generators are not ordered but compilation is (wrt dependencies), it could be worked around through separate projects each generating code which can be accessed through the metadata provider in the next generator, hosted as a separate project.

To do this each attributed type should be noted for efficient retrieval in the downstream project.

[GeneratorAttirbute] class A; // returned by ForAttributeWithMetadataName
[GeneratorAttirbute] class B; // returned by ForAttributeWithMetadataName

// generated
[assembly: Export(typeof(A)]
[assembly: Export(typeof(B)]

then the generator can combine symbols from both source and the metadata for processing.

If roslyn could add some sort of helper similar to ForAttributeWithMetadataName that does this, generators can choose to use it to support operating on generated code.