* 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.