`@[local ext]` should not generate a global `ext_iff` lemma

Prerequisites

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Description

Tagging a structure Foo with @[local ext] generates global Foo.ext and Foo.ext_iff. It seems better to generate them as private lemmas instead.

Context

Zulip

Steps to Reproduce

In file MWE:

@[local ext]
structure Foo where
  toNat := Nat

In another file,

import MWE

#check Foo.ext
#check Foo.ext_iff

Expected behavior: The #checks fail

Actual behavior: They succeed

Versions

4.14.0-rc2
Ubuntu

Impact

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Reproduce the behavior using the MWE file and the separate file containing the import and #check commands. Trace the handling of the local ext attribute, then verify that the #check commands fail after the change while local ext functionality still works.