Introduce phases for environment shell snippet in xbps-packages.

The environment shell snippets (available in xbps-packages/common/environment)
are now read only for the specified phases, being:

        * setup         (before reading template)
        * fetch         (before running fetch phase)
        * extract       (before running extract phase)
        * configure     (before running configure phase)
        * build         (before running build phase)
        * install       (before running install phase)
        * pkg           (before running pkg phase)

By making symlinks we can make a snippet be available at multiple phases.