refactor(sessionresolver): minor changes in files and types naming (#810)

Part of https://github.com/ooni/probe/issues/2135

internal/engine/internal/sessionresolver/errwrapper_test.go

@@ -9,10 +9,7 @@ import (
 )
 
 func TestErrWrapper(t *testing.T) {
-	ew := &errwrapper{
-		error: io.EOF,
-		URL:   "https://dns.quad9.net/dns-query",
-	}
+	ew := newErrWrapper(io.EOF, "https://dns.quad9.net/dns-query")
 	o := ew.Error()
 	expect := "<https://dns.quad9.net/dns-query> EOF"
 	if diff := cmp.Diff(expect, o); diff != "" {
@@ -21,4 +18,7 @@ func TestErrWrapper(t *testing.T) {
 	if !errors.Is(ew, io.EOF) {
 		t.Fatal("not the sub-error we expected")
 	}
+	if errors.Unwrap(ew) != io.EOF {
+		t.Fatal("unwrap failed")
+	}
 }