We handle it as follow:
Say, the loading project as file A and its proxy A.proxy

 - In Project::missing-uri, return the proxy target URI so the proxy
   is, proxied by it target (A.proxy will be proxied by A)
 - As soon as the A asset is ready, we start creating its proxy
 - Once the A.proxy is created, we reload it, unproxy it (to avoid proxy
   cycles), and start using it as a proxy for A

Also fix several places where we were considering that an asset
with a ->proxy_target != None was a proxy in our terms, it is not true
anymore as during the time where we are recreating 'A.proxy',
A.props.proxy_target is actually A.proxy, but it is no a proxy for us at
that point (just a temporary redirection).

Fixes T7560

Reviewed-by: Alex Băluț <&lt;alexandru.balut@gmail.com&gt;>
Differential Revision: https://phabricator.freedesktop.org/D1815