bug#33512: 26; Be able to specify soft-requires in file header (`Package-Requires:')

[Drew Adams <drew.adams@oracle.com>]

Please consider the enhancement of being able to specify, in a Lisp file
header, something like `Package-Requires' (or a `Package-Requires' that
allows variant arguments to accomplish the same thing), which would
specify that another package is soft-required, not hard-required.

That is, the requirement of another package, e.g. `foo.el', would be
akin to (require 'foo nil t), as opposed to just (require 'foo).

This would enable a package to advertise, for `package.el' and ELPA
repositories, that some of its features require another package, but
that that package is not strictly necessary (e.g., some of the requiring
package features are available without the soft-required package).

It might also encourage modularity and discourage adding extra stuff to
a given package (even duplicating some code from another package, for
instance).

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''