The api module previously had defaultPartition and defaultNamespace vars
for when we need default/empty split usage between ent/oss respectively.
This commit moves those two variables out of test code so that they can
be used for the service exports config entry's `GetNamespace()` method.
Previously `GetNamespace()` would return "default" in both OSS and enterprise,
which can trip up automation that passes the result of this method as the
namespace to write a config entry.
The split vars are kept private to prevent external usage, and prefixed with
`split` for more clarity about their behavior.
Existing config entries prefixed by service- are specific to individual
services. Since this config entry applies to partitions it is being
renamed.
Additionally, the Partition label was changed to Name because using
Partition at the top-level and in the enterprise meta was leading to the
enterprise meta partition being dropped by msgpack.
