Add compiler interfaces and build definition skeleton

Create compiler interface with switchable backends. For now it can resolve and
parse a `build.yaml` which is in the same folder as in `definition.yaml`

Add also tests to cover this and new fixtures

pkg/compiler/spec.go

@@ -0,0 +1,38 @@
+// Copyright © 2019 Ettore Di Giacinto <mudler@gentoo.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 compiler
+
+import (
+	yaml "gopkg.in/yaml.v2"
+)
+
+type LuetCompilationSpec struct {
+	Steps []string `json:"steps"`
+	Image string   `json:"image"`
+}
+
+func NewLuetCompilationSpec(b []byte) (CompilationSpec, error) {
+	var spec LuetCompilationSpec
+	err := yaml.Unmarshal(b, &spec)
+	if err != nil {
+		return &spec, err
+	}
+	return &spec, nil
+}
+
+func (cs *LuetCompilationSpec) ToDocker() (string, error) {
+	return "", nil
+}