Config: fix don't create empty dir when resolving path (#575)

* Config: fix don't create empty dir when resolving path

* refactor migration of default config file

* gracefully handle src == target on Move
* create target directory on Move, if necessary

* unify resolution of default config location

* refactor the use of flagSet to be explicit
* remove package variable
* use empty config file path setting if not set in flags
* resolve default file location the same way when showing the target and
when loading default config file

* don't migrate if target file is the same as source

* rename configfile to configFile

* add migrateConfig tests

gctscript/wrappers/gct/exchange/exchange_test.go

@@ -181,7 +181,7 @@ func TestExchange_CancelOrder(t *testing.T) {
 }
 
 func setupEngine() (err error) {
-	engine.Bot, err = engine.NewFromSettings(&settings)
+	engine.Bot, err = engine.NewFromSettings(&settings, nil)
 	if err != nil {
 		return err
 	}