* lisp/emacs-lisp/package.el: Fix decoding of downloaded files

This is a different fix for bug#34909, which should also fix bug#35739.

Our downloading code used to automatically decode the result according
to the usual heuristics for files.  This caused problems when we later
needed to save the data in a file that needed to be byte-for-byte
equal to the original in order to pass the signature verification,
especially because we didn't keep track of which coding-system was
used to decode the data.

(package--unless-error): New macro extracted from
package--with-response-buffer-1, so that we can specify edebug and
indent specs.
(package--with-response-buffer-1): Use it.  More importantly, change
code so it runs `body` in a unibyte buffer with undecoded data.
(package--download-one-archive): Don't encode with utf-8 since the data
is not decoded yet.
(describe-package-1): Explicitly decode the readem.txt files here.

* lisp/url/url-handlers.el (url-insert-file-contents): Use it.
(url-insert): Don't decode if buffer is unibyte.

* lisp/url/url-http.el (url-http--insert-file-helper): New function,
extracted from url-insert-file-contents.
1 job for master in 50 minutes and 30 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1827


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1