MacroSystem should handle fully qualified macro name

[ApolloZhu]

Description

When unit testing macro expansions using SwiftSyntaxMacroExpansion, there's no way for it to expand fully qualified attached macro

swift-syntax/Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

Line 997 in 2a731fb

let attributeName = attribute.attributeName.as(IdentifierTypeSyntax.self)?.name.text,

Steps to Reproduce

Try to write a unit test that expands a fully qualified attached macro:

@MyModule.MyMacro var hello = "world"

Neither of the following in the dictionary works with the SyntaxProtocol.expand function

[
    "MyMacro": MyMacro.self,
    "MyModule.MyMacro": MyMacro.self,
]

[ApolloZhu]

Tracked in Apple’s issue tracker as rdar://137364509