Fix a typo in legacy_enum.go.
Rename package in ancient legacy proto so that it doesn't confuse
tooling that assume that the package and directory names match.
Change-Id: I0b896045e74b0a7f998d3e5693b853eb3aa3839c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214182
Reviewed-by: Joe Tsai <joetsai@google.com>