service,terminal,cmd/dlv: automatically guessing substitute-path config (#3781)

Add command, API calls and launch.json option to automatically guess
substitute-path configuration.

service/api/types.go

@@ -677,3 +677,10 @@ type Target struct {
 	CmdLine       string
 	CurrentThread *Thread
 }
+
+// GuessSubstitutePathIn are the input parameters used to guess a substitute-path configuration automatically.
+type GuessSubstitutePathIn struct {
+	ImportPathOfMainPackage string
+	ClientGOROOT            string
+	ClientModuleDirectories map[string]string
+}