classes/yocto-check-layer: add check for tasks that allow network access

classes/yocto-check-layer: add check for tasks that allow network access

Add a new test that checks that no tasks between do_fetch (exclusive)
and do_build (inclusive) are allowed to use the network, with rare
exceptions.

The only exception currently is build-appliance-image's do_image task,
as that currently usese pip to install the required Toaster dependencies.

Note that this will mean layers that have Go-based recipes will fail
unless they're using the gomod fetcher and have a complete list of
modules in the SRC_URI.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>