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/compiler.go

@@ -0,0 +1,58 @@
+// 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 (
+	"errors"
+	"io/ioutil"
+
+	"github.com/mudler/luet/pkg/helpers"
+	pkg "github.com/mudler/luet/pkg/package"
+	"github.com/mudler/luet/pkg/tree"
+)
+
+const BuildFile = "build.yaml"
+
+type LuetCompiler struct {
+	*tree.Recipe
+	Backend CompilerBackend
+}
+
+func NewLuetCompiler(backend CompilerBackend, t pkg.Tree) Compiler {
+	return &LuetCompiler{Backend: backend, Recipe: &tree.Recipe{PackageTree: t}}
+}
+
+func (cs *LuetCompiler) Compile(p CompilationSpec) (*Artifact, error) {
+	return nil, errors.New("Not implemented yet")
+}
+
+func (cs *LuetCompiler) FromPackage(p pkg.Package) (CompilationSpec, error) {
+
+	pack, err := cs.Tree().GetPackageSet().FindPackage(p)
+	if err != nil {
+		return nil, err
+	}
+	buildFile := pack.Rel(BuildFile)
+	if !helpers.Exists(buildFile) {
+		return nil, errors.New("No build file present for " + p.GetFingerPrint())
+	}
+
+	dat, err := ioutil.ReadFile(buildFile)
+	if err != nil {
+		return nil, err
+	}
+	return NewLuetCompilationSpec(dat)
+}