lang: Pass through the fs and be consistent in usage

This simplifies the API by passing through the filesystem so that
function signatures don't need to be as complicated, and furthermore use
that consistently throughout.

lang/lang.go

@@ -139,9 +139,9 @@ func (obj *Lang) Init() error {
 	// init and validate the structure of the AST
 	data := &interfaces.Data{
 		// TODO: add missing fields here if/when needed
-		Fs:       obj.Fs,
-		FsURI:    obj.FsURI,
-		Base:     output.Base, // base dir (absolute path) the metadata file is in
+		Fs:       output.FS,       // formerly: obj.Fs
+		FsURI:    output.FS.URI(), // formerly: obj.FsURI
+		Base:     output.Base,     // base dir (absolute path) the metadata file is in
 		Files:    output.Files,
 		Imports:  importVertex,
 		Metadata: output.Metadata,