Supporting multiline Package-Requires header

[Artur Malabarba]

I'd like to add support for multiline Package-Requires headers in
package.el. Does anyone have any preference over the following 2
possible formats?

1. Same as now, just allow it to span more than one line. E.g.:

        ;; Package-Requires: ((pkg-a "4.2.0")
                              (pkg-b "1.0") (pkg-c "2.0"))

2. Without parenthesis, with exactly one per line. E.g.:

        ;; Package-Requires: pkg-a "4.2.0"
                             pkg-b "1.0"
                             pkg-c "2.0"


Of course, the current way will still be supported. For reference,
here is the current method.

    ;; Package-Requires: ((pkg-a "4.2.0") (pkg-b "1.0") (pkg-c "2.0"))