internal/validate: relocate from app

These are free of the dispatcher from internal/app. This change relocates them into their own package.

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

internal/validate/validate.go

@@ -0,0 +1,20 @@
+// Package validate provides functions for validating string values of various types.
+package validate
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// DeepContainsH returns whether basepath is equivalent to or is the parent of targpath.
+//
+// This is used for path hiding warning behaviour, the purpose of which is to improve
+// user experience and is *not* a security feature and must not be treated as such.
+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, upper), err
+}