Use get_network in populate_network function

[spacedmonkey]

In populate network there is a raw call to the sites table. This can easily be replaced with a get_network.

[flixos90]

@spacedmonkey Good catch! 41805.2.diff​ uses get_network() conditionally. This is needed because populate_network() may also be used when multisite is being set up and the multisite API is not yet available.

[flixos90]

In 41348:

Multisite: Use get_network() in populate_network() to check whether a network with the given ID already exists.

When multisite is setup already, e.g. in a multi network environment, this change gives a performance benefit over the direct SQL query that was previously used. The SQL query remains in place for when setting up multisite initially as the network API is not available at that point.

Props spacedmonkey.
Fixes #41805.