Add "hugo mod npm pack"

This commit also introduces a convention where these common JS config files, including `package.hugo.json`, gets mounted into:

```
assets/_jsconfig
´``

These files mapped to their real filename will be added to the environment when running PostCSS, Babel etc., so you can do `process.env.HUGO_FILE_TAILWIND_CONFIG_JS` to resolve the real filename.

But do note that `assets` is a composite/union filesystem, so if your config file is not meant to be overridden, name them something specific.

This commit also adds adds `workDir/node_modules` to `NODE_PATH` and `HUGO_WORKDIR` to the env when running the JS tools above.

Fixes #7644
Fixes #7656
Fixes #7675

commands/mod_npm.go

@@ -0,0 +1,58 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// 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 commands
+
+import (
+	"github.com/gohugoio/hugo/hugolib"
+	"github.com/gohugoio/hugo/modules/npm"
+	"github.com/spf13/cobra"
+)
+
+func newModNPMCmd(c *modCmd) *cobra.Command {
+
+	cmd := &cobra.Command{
+		Use:   "npm",
+		Short: "Various npm helpers.",
+		Long:  `Various npm (Node package manager) helpers.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return c.withHugo(func(h *hugolib.HugoSites) error {
+				return nil
+			})
+		},
+	}
+
+	cmd.AddCommand(&cobra.Command{
+		Use:   "pack",
+		Short: "Experimental: Prepares and writes a composite package.json file for your project.",
+		Long: `Prepares and writes a composite package.json file for your project.
+
+On first run it creates a "package.hugo.json" in the project root if not alread there. This file will be used as a template file
+with the base dependency set. 
+
+This set will be merged with all "package.hugo.json" files found in the dependency tree, picking the version closest to the project.
+
+This command is marked as 'Experimental'. We think it's a great idea, so it's not likely to be
+removed from Hugo, but we need to test this out in "real life" to get a feel of it,
+so this may/will change in future versions of Hugo.
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+
+			return c.withHugo(func(h *hugolib.HugoSites) error {
+				return npm.Pack(h.BaseFs.SourceFs, h.BaseFs.Assets.Dirs)
+			})
+		},
+	})
+
+	return cmd
+}