bug#66636: Move lexical-binding warning from checkdoc to byte-compiler

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> The warning about a missing lexical-binding cookie rather belongs in
>> the compiler than checkdoc, because it's not about documentation or
>> style but code generation and ability to detect errors, both which are
>> hindered by a missing cookie.
>>
>> Moving the warning to the compiler also makes it more widely seen.
>
> So long as this warning is only displayed within code part of Emacs
> itself, there are no valid objections to such a change.
>
> But you have instead elected to generate warnings whenever such files
> are byte-compiled.  There exist many packages which do not enable
> lexical binding, whose authors have studiously elected not to: most of
> Drew Adams' for example.  So this is tantamount to punitive action
> against their users, in the form of an unsightly warning each time such
> packages are installed.

No, it's just asking those authors to make the result of their studious
work explicit with

    -*- lexical-binding: nil -*-


-- Stefan