Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
This patch started by addresssing the code review comments to close
https://github.com/rycee/home-manager/pull/290. However initiating a new
pull request it became clear, that home-manager changed significantly
since then.
This changes the initial pull request to be consistent with the email
account management in home-manager now. It also adds a simple test and support
for multiple accounts.
