Fix method signatures being overwritten by prepended modules

[DaneHarrison]

Motivation

Fixes #2072 - When a module is prepended onto a class, Tapioca generates the RBI using the prepended module's signature instead of its own.

Implementation

compile_method receives methods obtained with Module#instance_method, which follows the ancestor chain. When a
module is prepended to a class and defines the method, instance_method returns the prepended module's UnboundMethod rather than the class's own, so compile_method was compiling the wrong method.

• method_owned_by_constant? becomes method_defined_by_constant. It walks super_method and returns the method the
  constant itself defines.

• New signature_defined_by_constant returns the signature declared on that method. A signature is only accepted
  if signature.method.owner matches the method's owner.

• The attr_accessor writer inference resolves the constant's own reader first, so it works in either evaluation order.

Further Considerations:

• If both the class and the prepended module have a sig for the same name, Sorbet keeps only one per key. Whichever block runs last wins - see wrap_method_if_needed and unwrap_method
• There are other callers to signature_of i.e under dsl, I suspect those will need to be updated as well
  • Let me know if you'd like me to do a follow up PR on this

Tests

• No sig: a prepended module in front of the method, and multiple prepended modules
• Cross-gem prepend matching the issue, checking both gems' RBIs
• Sig with a prepended module that has different parameters
• Only the prepended module has a sig (the class's method must not take it)
• Sig evaluated before the prepend, and between two prepends
• attr_accessor writer inference with the reader's sig evaluated before and after the prepend
• A prepended module's sig that fails to load is reported once, against the module
• Skipped: both the class and the prepended module have a sig (Sorbet keeps only one)

[DaneHarrison]

I have signed the CLA!