Add hook to allow lazy loading of import sources

Add hook to allow lazy loading of import sources

Currently import sources have to be set into $wgImportSources as part of
wiki startup. This is not practical for the WMF cluster, where we need some
reasonably complex logic to set up the import source structure.

This change allows the import source list to be populated from a new
"ImportSources" hook. This hook is only called when the list of import
sources is actually needed (namely, when a user with relevant permissions
loads Special:Import).

Bug: T17583
Change-Id: Ice9a19cb6dfe53ae72aa71353d0553ee9338f233