Update filesystem interface to be more consistent

This change updates `Open` to return a `ReadCloser` rather than a
`ReadWriteCloser`. This is because it opens the file as R_ONLY.

This change also updates the `Create` method to return a `WriteCloser`
to be consistent with the interface provided by `Open`.

[#152763277]

Signed-off-by: Andrew Crump <acrump@pivotal.io>

Author: jfmyers9

builder/fakes/filesystem.go

@@ -9,7 +9,7 @@ import (
 	"testing"
 )
 
-//go:generate counterfeiter -o ./fakes/read_write_closer.go --fake-name ReadWriteCloser io.ReadWriteCloser
+//go:generate counterfeiter -o ./fakes/read_closer.go --fake-name ReadCloser io.ReadCloser
 
 func TestBuilder(t *testing.T) {
 	RegisterFailHandler(Fail)

builder/fakes/read_write_closer.go builder/fakes/read_closer.gobuilder/fakes/read_write_closer.go renamed to builder/fakes/read_closer.go

@@ -20,37 +20,46 @@ func NewMetadataPartsDirectoryReader(filesystem filesystem, topLevelKey string)
 
 func (r MetadataPartsDirectoryReader) Read(path string) ([]interface{}, error) {
 	parts := []interface{}{}
+
 	err := r.filesystem.Walk(path, func(filePath string, info os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
+
 		if info.IsDir() {
 			return nil
 		}
+
 		if filepath.Ext(filePath) != ".yml" {
 			return nil
 		}
+
 		f, err := r.filesystem.Open(filePath)
 		if err != nil {
 			return err
 		}
 		defer f.Close()
+
 		data, err := ioutil.ReadAll(f)
 		if err != nil {
 			return err
 		}
+
 		var fileVars map[string][]interface{}
 		err = yaml.Unmarshal([]byte(data), &fileVars)
 		if err != nil {
 			return err
 		}
+
 		_, ok := fileVars[r.topLevelKey]
 		if !ok {
 			return fmt.Errorf("not a %s file: %q", r.topLevelKey, filePath)
 		}
+
 		parts = append(parts, fileVars[r.topLevelKey]...)
 		return nil
 	})
+
 	if err != nil {
 		return []interface{}{}, err
 	}