* Rename distributedcontext package to correlation
Correlation is the name we agreed upon.
* Move trace propagators to api/trace
The trace propagators tests had to be moved to a testtrace subpackage
to avoid import cycles between api/trace and internal/trace.
Needed to shut up golint about stutter in trace.TraceContext -
TraceContext is a name of a W3C spec, so this stutter is
expected. It's certainly still better than golint's suggestion of
having trace.Context.
* Rename api/propagators to api/propagation
This package will not contain any propagators in the long run, just
the interface definitions.
Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>
