Reorganize propagation code (shrink PR 381) (#444)

* 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>

example/http/client/client.go

@@ -25,7 +25,7 @@ import (
 
 	"google.golang.org/grpc/codes"
 
-	"go.opentelemetry.io/otel/api/distributedcontext"
+	"go.opentelemetry.io/otel/api/correlation"
 	"go.opentelemetry.io/otel/api/global"
 	"go.opentelemetry.io/otel/api/key"
 	"go.opentelemetry.io/otel/api/trace"
@@ -56,7 +56,7 @@ func main() {
 	initTracer()
 
 	client := http.DefaultClient
-	ctx := distributedcontext.NewContext(context.Background(),
+	ctx := correlation.NewContext(context.Background(),
 		key.String("username", "donuts"),
 	)
 

example/http/server/server.go

@@ -19,7 +19,7 @@ import (
 	"log"
 	"net/http"
 
-	"go.opentelemetry.io/otel/api/distributedcontext"
+	"go.opentelemetry.io/otel/api/correlation"
 	"go.opentelemetry.io/otel/api/global"
 	"go.opentelemetry.io/otel/api/trace"
 	"go.opentelemetry.io/otel/exporter/trace/stdout"
@@ -52,7 +52,7 @@ func main() {
 	helloHandler := func(w http.ResponseWriter, req *http.Request) {
 		attrs, entries, spanCtx := httptrace.Extract(req.Context(), req)
 
-		req = req.WithContext(distributedcontext.WithMap(req.Context(), distributedcontext.NewMap(distributedcontext.MapUpdate{
+		req = req.WithContext(correlation.WithMap(req.Context(), correlation.NewMap(correlation.MapUpdate{
 			MultiKV: entries,
 		})))