perf: skip walking unnecessary dirs when reading previous generation

Noticed an error like this after running truss with a flattened service
path.
```
truss --svcout . service.proto
FATA[0000] cannot parse input: cannot read previously generated files: cannot fully walk directory /home/zaq/projects/go-mod/src/github.com/<redacted>: cannot read file: /home/zaq/projects/go-mod/src/github.com/<redacted>/.git/refs/remotes/origin/<redacted>: open /home/zaq/projects/go-mod/src/github.com/<redacted>/.git/refs/remotes/origin/<some_branch>: too many open files
```
NOTE: was using default nofiles limit on linux
```
$ ulimit -Sn
1024
```
When using `--svcout .`, `cmd`, `svc`, and `handlers` folder will
be created in the current directory. Which is really nice if you want a
flatter service structure. However, this has the unintented consequence
of opening significantly more files when truss is building context on
it's previous generation. This change cuts down the files read to only
those in the handlers dir as these are the only files whose previous
generation is leveraged

Ref: `gengokit/generator/gen.go:generateResponseFile`

cmd/truss/main.go

@@ -359,15 +359,23 @@ func readPreviousGeneration(serviceDir string) (map[string]io.Reader, error) {
 		return nil, nil
 	}
 
+	const handlersDirName = "handlers"
 	files := make(map[string]io.Reader)
 
 	addFileToFiles := func(path string, info os.FileInfo, err error) error {
 		if info.IsDir() {
-			return nil
+			switch info.Name() {
+			// Only files within the handlers dir are used to
+			// support regeneration.
+			// See `gengokit/generator/gen.go:generateResponseFile`
+			case filepath.Base(serviceDir), handlersDirName:
+				return nil
+			default:
+				return filepath.SkipDir
+			}
 		}
 
 		file, ioErr := os.Open(path)
-
 		if ioErr != nil {
 			return errors.Wrapf(ioErr, "cannot read file: %v", path)
 		}