Refactor manifests generation

- introduce manifestgen pkg, to be consumed by the CLI and Terraform provider
- consolidate defaults in manifestgen/install pkg
- introduce Manifest as the returning type of manifest generation
- add helper function to Manifest for writing multi-doc YAMLs on disk

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

pkg/manifestgen/install/install.go

@@ -0,0 +1,72 @@
+/*
+Copyright 2020 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package install
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"strings"
+
+	"github.com/fluxcd/toolkit/pkg/manifestgen"
+)
+
+// Generate returns the install manifests as a multi-doc YAML.
+// The manifests are built from a GitHub release or from a
+// Kustomize overlay if the supplied Options.BaseURL is a local path.
+func Generate(options Options) (*manifestgen.Manifest, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), options.Timeout)
+	defer cancel()
+
+	tmpDir, err := ioutil.TempDir("", options.Namespace)
+	if err != nil {
+		return nil, fmt.Errorf("temp dir error: %w", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	output := path.Join(tmpDir, options.ManifestFile)
+
+	if !strings.HasPrefix(options.BaseURL, "http") {
+		if err := build(options.BaseURL, output); err != nil {
+			return nil, err
+		}
+	} else {
+		if err := fetch(ctx, options.BaseURL, options.Version, tmpDir); err != nil {
+			return nil, err
+		}
+
+		if err := generate(tmpDir, options); err != nil {
+			return nil, err
+		}
+
+		if err := build(tmpDir, output); err != nil {
+			return nil, err
+		}
+	}
+
+	content, err := ioutil.ReadFile(output)
+	if err != nil {
+		return nil, err
+	}
+
+	return &manifestgen.Manifest{
+		Path:    path.Join(options.TargetPath, options.Namespace, options.ManifestFile),
+		Content: string(content),
+	}, nil
+}