Support invoking Scala 2 macros in a Scala 3 binary

[bishabosha]

[open]

• find how Scala 2 discovers a macro implementation and how to link to it
• ensure dotty publishes that information alongside a Scala 2 macro definition in TASTy

[bishabosha]

This seems more tricky because the tasty reader would need access to an Analyzer to run the standard macro linking implementation. Either we can move the code for checking format of a macro def to symbol table, or ClassfileParser will also need access to an Analyzer to provide it to the Tasty unpickler