Use `native.package_relative_label` in `selects.bzl` (#576)

Prepares for a future in which `select` may eagerly resolve string keys to `Label`s.

Co-authored-by: Xùdōng Yáng <wyverald@gmail.com>

lib/selects.bzl

diff --git a/lib/selects.bzl b/lib/selects.bzl
index c511fe5..0c29215 100644
--- a/lib/selects.bzl
+++ b/lib/selects.bzl
@@ -178,7 +178,7 @@
         native.alias(
             name = name if i == 1 else name + "_" + str(i),
             actual = select({
-                settings[i - 1]: settings[i - 1],
+                native.package_relative_label(settings[i - 1]): settings[i - 1],
                 "//conditions:default": actual[i - 1],
             }),
             visibility = visibility if i == 1 else ["//visibility:private"],
@@ -218,7 +218,7 @@
         native.alias(
             name = name if i == 1 else name + "_" + str(i),
             actual = select({
-                settings[i - 1]: actual[i - 1],
+                native.package_relative_label(settings[i - 1]): actual[i - 1],
                 "//conditions:default": settings[i - 1],
             }),
             visibility = visibility if i == 1 else ["//visibility:private"],