reduce number of open files while processing nested java archives (#227)

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/opener.go

@@ -0,0 +1,14 @@
+package file
+
+import (
+	"io"
+	"os"
+)
+
+type Opener struct {
+	path string
+}
+
+func (o Opener) Open() (io.ReadCloser, error) {
+	return os.Open(o.path)
+}