internal/app: declutter and merge small files

This should make internal/app easier to work with for the upcoming params to shim.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/path.go

@@ -6,6 +6,10 @@ import (
 )
 
 func deepContainsH(basepath, targpath string) (bool, error) {
+	const upper = ".." + string(filepath.Separator)
+
 	rel, err := filepath.Rel(basepath, targpath)
-	return err == nil && rel != ".." && !strings.HasPrefix(rel, string([]byte{'.', '.', filepath.Separator})), err
+	return err == nil &&
+		rel != ".." &&
+		!strings.HasPrefix(rel, upper), err
 }