fix: enable self-referencing schemas

[eliax1996]

Before this PR the protobuf schema was able to refer only to fully qualified names or names defined in an higher leaf of the parsing tree.
This commit enable a schema to refer the same scope level if at the same leaf level there is defined exactly the field required after.

In an example:

enum FancyEnum {
  ...
}
message FancyMessage1 {
   FancyEnum field ...
}

FancyMessage1 is allowed to call FancyEnum because his scope is /FancyMessage1 while the scope of FancyEnum it's / (the name itself doesn't count in the declarations, but take part only in field assignment inside the declarations, we could say that field has the scope set to /FancyMessage1 but the type FancyEnum has also the scope /, only when referring to a type you enter in it's scope during the parsing procedure).

This was a valid declaration in the previous status.

Instead, in this example:

message FancyMessage1 {
   enum FancyEnum {
     ...
   }
   FancyEnum field ...
}

In the previous state was an invalid schema.

With this pr, since FancyEnum field ... it's declared inside the scope /FancyMessage1 but also the requested type FancyEnum it's declared in the same scope, /FancyMessage1 it's considered valid

Fixes #713

[tvainika]

This code still fails with slightly modified schema like

syntax = "proto3";

package fancy.company.in.party.v1;
message AnotherMessage {
  enum BamFancyEnum {
    // Hei! This is a comment!
    MY_AWESOME_FIELD = 0;
  }
  message WowANestedMessage {
    message DeeplyNestedMsg {
      BamFancyEnum im_tricky_im_referring_to_the_previous_enum = 1;
    }
  }
}

which I believe is as valid as your test case.

[juha-aiven]

In description

...the avro schema was able to refer ...

You mean of course protobuf schema.