Flip objc_(import|library) to Starlark

This will start using the Starlark version of the rule's by default.

TGP is clean but if problems become apparent after a release, in order to avoid
a rollback, you can add --experimental_builtins_injection_override=-objc_library,-objc_import
to the blazerc.

Performance regressions from 9a423f8 are fixed. See Benchmark chip.

TESTED=[]

RELNOTES:none
PiperOrigin-RevId: 382566869

src/main/starlark/builtins_bzl/common/exports.bzl

@@ -24,7 +24,7 @@ exported_toplevels = {
     "_builtins_dummy": "overridden value",
 }
 exported_rules = {
-    "-objc_import": objc_import,
-    "-objc_library": objc_library,
+    "+objc_import": objc_import,
+    "+objc_library": objc_library,
 }
 exported_to_java = {}