This helper method is only really used for testing, but we should not
allow it to create a Key who's namespace has a colon (as ParseKey
would not build something like this).
This API was introduced to aid the refactoring, but it isn't something
we want to support long term. Remove it and for a few places, push
passing config.Key around more, instead of converting to the old type
eagerly.
When serializing config.Key's we now write them as <package>:<name>
instead of <package>:config:<name>. We continue to support reading the
older format for compatability with older files.
config.Key has become a pair of namespace and name. Because the whole
world has not changed yet, there continues to be a way to convert
between a tokens.ModuleMember and config.Key, however now sometime the
conversion from tokens.ModuleMember can fail (when the module member
is not of the form `<package>:config:<name>`).
