Make js.Build fully support modules

Fixes #7816
Fixes #7777
Fixes #7916

hugofs/fileinfo.go

@@ -37,6 +37,7 @@ import (
 const (
 	metaKeyFilename = "filename"
 
+	metaKeySourceRoot                 = "sourceRoot"
 	metaKeyBaseDir                    = "baseDir" // Abs base directory of source file.
 	metaKeyMountRoot                  = "mountRoot"
 	metaKeyModule                     = "module"
@@ -128,6 +129,10 @@ func (f FileMeta) PathFile() string {
 	return strings.TrimPrefix(strings.TrimPrefix(f.Filename(), base), filepathSeparator)
 }
 
+func (f FileMeta) SourceRoot() string {
+	return f.stringV(metaKeySourceRoot)
+}
+
 func (f FileMeta) MountRoot() string {
 	return f.stringV(metaKeyMountRoot)
 }

hugofs/rootmapping_fs.go

@@ -60,6 +60,7 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 			rm.Meta = make(FileMeta)
 		}
 
+		rm.Meta[metaKeySourceRoot] = rm.To
 		rm.Meta[metaKeyBaseDir] = rm.ToBasedir
 		rm.Meta[metaKeyMountRoot] = rm.path
 		rm.Meta[metaKeyModule] = rm.Module