🎨 Introduce contextualized logging

This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.

Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.

Closes #265

pkg/api/core/garbagecollector/garbagecollector.go

@@ -0,0 +1,59 @@
+// Copyright © 2021 Ettore Di Giacinto <mudler@mocaccino.org>
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <http://www.gnu.org/licenses/>.
+
+package gc
+
+import (
+	"io/ioutil"
+	"os"
+)
+
+type GarbageCollector string
+
+func (c GarbageCollector) String() string {
+	return string(c)
+}
+
+func (c GarbageCollector) init() error {
+	if _, err := os.Stat(string(c)); err != nil {
+		if os.IsNotExist(err) {
+			err = os.MkdirAll(string(c), os.ModePerm)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (c GarbageCollector) Clean() error {
+	return os.RemoveAll(string(c))
+}
+
+func (c GarbageCollector) TempDir(pattern string) (string, error) {
+	err := c.init()
+	if err != nil {
+		return "", err
+	}
+	return ioutil.TempDir(string(c), pattern)
+}
+
+func (c GarbageCollector) TempFile(s string) (*os.File, error) {
+	err := c.init()
+	if err != nil {
+		return nil, err
+	}
+	return ioutil.TempFile(string(c), s)
+}