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/inputs/inputs.go

@@ -66,9 +66,10 @@ var (
 // ParsedInput is the output struct which contains all the information we need.
 type ParsedInput struct {
 	//activated bool // if struct is not nil we're activated
-	Base     string   // base path (abs path with trailing slash)
-	Main     []byte   // contents of main entry mcl code
-	Files    []string // files and dirs to copy to fs (abs paths)
+	FS       engine.Fs // reference to the engine.Fs used to call the parse
+	Base     string    // base path (abs path with trailing slash)
+	Main     []byte    // contents of main entry mcl code
+	Files    []string  // files and dirs to copy to fs (abs paths)
 	Metadata *interfaces.Metadata
 	Workers  []func(engine.Fs) error // copy files here that aren't listed!
 }
@@ -214,6 +215,7 @@ func inputMetadata(s string, fs engine.Fs) (*ParsedInput, error) {
 		return nil, errwrap.Wrapf(err, "could not build metadata")
 	}
 	return &ParsedInput{
+		FS:       fs,
 		Base:     basePath,
 		Main:     b,
 		Files:    files,
@@ -261,6 +263,7 @@ func inputMcl(s string, fs engine.Fs) (*ParsedInput, error) {
 		},
 	}
 	return &ParsedInput{
+		FS:   fs,
 		Base: dirify(filepath.Dir(s)), // base path with trailing slash
 		Main: b,
 		Files: []string{
@@ -363,6 +366,7 @@ func inputCode(s string, fs engine.Fs) (*ParsedInput, error) {
 	}
 
 	return &ParsedInput{
+		FS:       fs,
 		Base:     dirify(wd),
 		Main:     b,
 		Files:    []string{}, // they're already copied in