Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

cache/filecache/filecache.go

@@ -35,7 +35,7 @@ import (
 var ErrFatal = errors.New("fatal filecache error")
 
 const (
-	filecacheRootDirname = "filecache"
+	FilecacheRootDirname = "filecache"
 )
 
 // Cache caches a set of files in a directory. This is usually a file on
@@ -301,7 +301,7 @@ func (c *Cache) isExpired(modTime time.Time) bool {
 }
 
 // For testing
-func (c *Cache) getString(id string) string {
+func (c *Cache) GetString(id string) string {
 	id = cleanID(id)
 
 	c.nlocker.Lock(id)
@@ -328,38 +328,24 @@ func (f Caches) Get(name string) *Cache {
 // NewCaches creates a new set of file caches from the given
 // configuration.
 func NewCaches(p *helpers.PathSpec) (Caches, error) {
-	var dcfg Configs
-	if c, ok := p.Cfg.Get("filecacheConfigs").(Configs); ok {
-		dcfg = c
-	} else {
-		var err error
-		dcfg, err = DecodeConfig(p.Fs.Source, p.Cfg)
-		if err != nil {
-			return nil, err
-		}
-	}
-
+	dcfg := p.Cfg.GetConfigSection("caches").(Configs)
 	fs := p.Fs.Source
 
 	m := make(Caches)
 	for k, v := range dcfg {
 		var cfs afero.Fs
 
-		if v.isResourceDir {
+		if v.IsResourceDir {
 			cfs = p.BaseFs.ResourcesCache
 		} else {
 			cfs = fs
 		}
 
 		if cfs == nil {
-			// TODO(bep) we still have some places that do not initialize the
-			// full dependencies of a site, e.g. the import Jekyll command.
-			// That command does not need these caches, so let us just continue
-			// for now.
-			continue
+			panic("nil fs")
 		}
 
-		baseDir := v.Dir
+		baseDir := v.DirCompiled
 
 		if err := cfs.MkdirAll(baseDir, 0777); err != nil && !os.IsExist(err) {
 			return nil, err
@@ -368,7 +354,7 @@ func NewCaches(p *helpers.PathSpec) (Caches, error) {
 		bfs := afero.NewBasePathFs(cfs, baseDir)
 
 		var pruneAllRootDir string
-		if k == cacheKeyModules {
+		if k == CacheKeyModules {
 			pruneAllRootDir = "pkg"
 		}