Add feature to get target Emacs platform from Cask/pkg headers

[Fuco1]

If we are analyzing a project which uses Cask or has a -pkg file or the requires headers we should pick the emacs dependency version and warn if user uses functions not supported by that platform.

package-lint has a list of these by version: https://github.com/purcell/package-lint/blob/master/package-lint.el#L83

• verify those lists are complete
• learn to find and read Cask
• learn to find and read -pkg.el file
• learn to find and read the Package-requires: header
• in case none of the above apply we should be able to specify (or override) the target in Elsafile.

[ebpa]

IMO, It would be intrinsically useful to programmatically examine package dependencies (beyond just the declared emacs version).

Some fragments that might be useful to this end:

• learn to find and read Cask

(locate-dominating-file default-directory cask-filename)

• learn to find and read -pkg.el file

(with-current-buffer (find-file (package--description-file default-directory))
  (package--read-pkg-desc 'single))

• learn to find and read the Package-requires: header

(package-desc-reqs (package-buffer-info))
;; OR
(lm-header "Package-Requires")

Edit: I've found the lisp-mnt.el functions extremely helpful.