internal/impl: rename legacy_extension.go as legacy_extension_hack.go

The current logic in this file is temporary and will be removed.
Rename the file as legacy_extension_hack.go so that the future
legacy_extension.go file can contain the actual legacy code that cannot be
removed anytime in the near future.

Change-Id: I7edd9d682836379c4c97d18ea2c2a2e3b15db5a2
Reviewed-on: https://go-review.googlesource.com/c/147918
Reviewed-by: Herbie Ong <herbie@google.com>