…replacements (#607)

…r.FindFieldByName instead of fmt.Sprintf (#611)

If given `protoregistry.GlobalFiles` and `protoregistry.GlobalTypes`,
this allows it to fallback to locally known descriptors if a server is
unable to provide part of the schema.

…621)

This resolves #620.

The `desc/sourceinfo` package previously wrapped descriptors, by
returning an implementation of the various `protoreflect.Descriptor`
interfaces that _embedded_ the original descriptor values. However,
these various interfaces are all defined as "[do not
implement](https://github.com/protocolbuffers/protobuf-go/blob/c33baa8f3a0d35fd5a39e43c22a50a050f707d34/reflect/protoreflect/type.go#L111)".
So, to avoid future issues where the protobuf runtime may type-assert
implementations to their own internal types (which would panic/fail with
the concrete wrapper types in this package), we no longer wrap this way.

This also happens to resolve #618 (since there's no more wrapping, an
incorrect typed-nil wrapper can't escape).