This is change 5/5 in a series of commits changing protoV1.ExtensionDesc
to directly implement protoreflect.ExtensionType.
1. [v2] Add protoimpl.ExtensionInfo as an alias for
protoiface.ExtensionDescV1.
2. [v1] Update references to protoimpl.ExtensionInfo to use
protoiface.ExtensionInfo.
3. [v2] Create protoimpl.ExtensionInfo (an alias to a new type in
the impl package) and remove protoiface.ExtensionDescV1.
4. [v1] Remove unneeded explicit conversions between ExtensionDesc and
ExtensionType (since the former now directly implements the latter).
5. [v2] Remove stub conversion functions.
Change-Id: I1fdfb18c481e72a362a6f9ee0e440f8f909790ca
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189564
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>