pkg: hide it by moving to _pkg.dev

The idea here is to preserve the history of `dev` branch development and its
code when merging with the `master`. Later this code could be moved into the
masters code where appropriate.

_pkg.dev/wire/payload/blockbase_test.go

@@ -0,0 +1,8 @@
+package payload
+
+import "testing"
+
+func Test(t *testing.T) {
+	//tests for this have been included in the mheaders_test file
+
+}