Add Hugo Modules

This commit implements Hugo Modules.

This is a broad subject, but some keywords include:

* A new `module` configuration section where you can import almost anything. You can configure both your own file mounts nd the file mounts of the modules you import. This is the new recommended way of configuring what you earlier put in `configDir`, `staticDir` etc. And it also allows you to mount folders in non-Hugo-projects, e.g. the `SCSS` folder in the Bootstrap GitHub project.
* A module consists of a set of mounts to the standard 7 component types in Hugo: `static`, `content`, `layouts`, `data`, `assets`, `i18n`, and `archetypes`. Yes, Theme Components can now include content, which should be very useful, especially in bigger multilingual projects.
* Modules not in your local file cache will be downloaded automatically and even "hot replaced" while the server is running.
* Hugo Modules supports and encourages semver versioned modules, and uses the minimal version selection algorithm to resolve versions.
* A new set of CLI commands are provided to manage all of this: `hugo mod init`,  `hugo mod get`,  `hugo mod graph`,  `hugo mod tidy`, and  `hugo mod vendor`.

All of the above is backed by Go Modules.

Fixes #5973
Fixes #5996
Fixes #6010
Fixes #5911
Fixes #5940
Fixes #6074
Fixes #6082
Fixes #6092

hugolib/fileInfo.go

@@ -16,82 +16,64 @@ package hugolib
 import (
 	"strings"
 
-	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugofs/files"
+
+	"github.com/pkg/errors"
+
+	"github.com/gohugoio/hugo/hugofs"
+
+	"github.com/spf13/afero"
+
 	"github.com/gohugoio/hugo/source"
 )
 
 // fileInfo implements the File and ReadableFile interface.
 var (
-	_ source.File         = (*fileInfo)(nil)
-	_ source.ReadableFile = (*fileInfo)(nil)
-	_ pathLangFile        = (*fileInfo)(nil)
+	_ source.File = (*fileInfo)(nil)
 )
 
-// A partial interface to prevent ambigous compiler error.
-type basePather interface {
-	Filename() string
-	RealName() string
-	BaseDir() string
-}
-
 type fileInfo struct {
-	bundleTp bundleDirType
-
-	source.ReadableFile
-	basePather
+	source.File
 
 	overriddenLang string
+}
 
-	// Set if the content language for this file is disabled.
-	disabled bool
+func (fi *fileInfo) Open() (afero.File, error) {
+	f, err := fi.FileInfo().Meta().Open()
+	if err != nil {
+		err = errors.Wrap(err, "fileInfo")
+	}
+
+	return f, err
 }
 
 func (fi *fileInfo) Lang() string {
 	if fi.overriddenLang != "" {
 		return fi.overriddenLang
 	}
-	return fi.ReadableFile.Lang()
-}
-
-func (fi *fileInfo) Filename() string {
-	if fi == nil || fi.basePather == nil {
-		return ""
-	}
-	return fi.basePather.Filename()
+	return fi.File.Lang()
 }
 
 func (fi *fileInfo) String() string {
-	if fi == nil || fi.ReadableFile == nil {
+	if fi == nil || fi.File == nil {
 		return ""
 	}
 	return fi.Path()
 }
 
-func (fi *fileInfo) isOwner() bool {
-	return fi.bundleTp > bundleNot
-}
+// TODO(bep) rename
+func newFileInfo(sp *source.SourceSpec, fi hugofs.FileMetaInfo) (*fileInfo, error) {
 
-func IsContentFile(filename string) bool {
-	return contentFileExtensionsSet[strings.TrimPrefix(helpers.Ext(filename), ".")]
-}
-
-func (fi *fileInfo) isContentFile() bool {
-	return contentFileExtensionsSet[fi.Ext()]
-}
-
-func newFileInfo(sp *source.SourceSpec, baseDir, filename string, fi pathLangFileFi, tp bundleDirType) *fileInfo {
-
-	baseFi := sp.NewFileInfo(baseDir, filename, tp == bundleLeaf, fi)
-	f := &fileInfo{
-		bundleTp:     tp,
-		ReadableFile: baseFi,
-		basePather:   fi,
+	baseFi, err := sp.NewFileInfo(fi)
+	if err != nil {
+		return nil, err
 	}
 
-	lang := f.Lang()
-	f.disabled = lang != "" && sp.DisabledLanguages[lang]
+	f := &fileInfo{
+		File: baseFi,
+	}
 
-	return f
+	return f, nil
 
 }
 
@@ -108,7 +90,7 @@ const (
 // Returns the given file's name's bundle type and whether it is a content
 // file or not.
 func classifyBundledFile(name string) (bundleDirType, bool) {
-	if !IsContentFile(name) {
+	if !files.IsContentFile(name) {
 		return bundleNot, false
 	}
 	if strings.HasPrefix(name, "_index.") {