[gentoo-dev] Allow variable refs in HOMEPAGE

[Mike Gilbert <floppym@gentoo.org>]

I would like to remove the ban on variable references in the HOMEPAGE
variable in ebuilds.

As I understand it, this ban was put in place so that people can
easily copy/paste from an ebuild to a web browser.

If people want to copy/paste the URL, they can easily query the final
value using the portage API, or a tool written against it.

% equery m www-client/chromium
 * www-client/chromium [gentoo]
Maintainer:  chromium@gentoo.org (Chromium in Gentoo Project)
Upstream:    None specified
Homepage:    http://chromium.org/
Location:    /home/floppym/repos/gentoo/www-client/chromium
Keywords:    59.0.3071.104:0: x86
Keywords:    60.0.3112.40:0:
Keywords:    60.0.3112.78:0: amd64
Keywords:    61.0.3159.5:0:
Keywords:    61.0.3163.13:0: ~amd64 ~arm ~arm64 ~x86
License:     BSD

Allowing variables in HOMEPAGE would allow for easier copying of
similar/identical ebuilds. For example, see the trivial ebuilds in my
overlay below.

https://bitbucket.org/floppym/floppym-overlay/src/8b55fab291ec1f9be93ade9c91e7d62d7b8a259d/dev-go/cloudflare-go/cloudflare-go-0.7.4.ebuild

https://bitbucket.org/floppym/floppym-overlay/src/8b55fab291ec1f9be93ade9c91e7d62d7b8a259d/dev-go/pkg-errors/pkg-errors-0.8.0.ebuild

https://bitbucket.org/floppym/floppym-overlay/src/8b55fab291ec1f9be93ade9c91e7d62d7b8a259d/dev-go/urfave-cli/urfave-cli-1.19.1.ebuild