Delay refreshing git submodule until we have a clone

Description

This should fix fallback download from mirrors when upstream is not available.

The way to fix it is to use a generator of fetchers rather than a list. Submodule fetchers are generated only one the main repository has been fetched.

There are also tests picked from !618 (closed)

This merge request, when approved, will close issue/bug: #537 (closed)

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