dir: Use .casengine-* temporary directories

Tool-specific filenames make tool-specific cleanup easier.  If a
casengine dir handler crashes or is abandoned without the Close()
cleanup, this lets you clean up after it with:

  $ rm -rf /path/to/base/.casengine-*

or similar without having to worry about blowing away non-casengine
content.  More discussion about this use-case and alternative
approaches in [1].

[1]: https://github.com/openSUSE/umoci/pull/198

Signed-off-by: W. Trevor King <wking@tremily.us>

dir/dir.go

@@ -84,7 +84,7 @@ func (r *RegexpGetDigest) GetDigest(path string) (dig digest.Digest, err error)
 
 // New creates a new CAS-engine instance.
 func New(ctx context.Context, path string, uri string, getDigest GetDigest) (engine casengine.Engine, err error) {
-	temp, err := ioutil.TempDir(path, "tmp-")
+	temp, err := ioutil.TempDir(path, ".casengine-")
 	if err != nil {
 		return nil, err
 	}