Add at_startup flag to lazy_import

[nthiery]

Lazy imports have two main use cases: (a) Delaying imports that are not always needed for a faster Sage startup (b) Resolving import loops

This ticket introduce an option "at_startup" to allow for lazy imports that are known to be resolved at startup time. The main use case is (b) in the context of #10963. E.g. the categories Magmas and Semigroups import each other reciprocally (because Magmas.Associative points to Semigroups). It is natural to resolve this by defining Magmas.Associative using a lazy import; one can then play with Magmas() without having to load Semigroups. However Semigroups() is later constructed during Sage's startup (and this is unlikely to change).

This option could also be used as an intermediate step toward (a), when a module is used in several places and some of those places can already afford to lazy import the module but not yet others. As a bonus, a lazy import that is marked as "at_startup" will print a message if it is actually resolved after the startup, so that the developer know that (s)he can remove the flag.

[tscrim]

I'm happy with the patch; are there any objections?

Best,
Travis

[tscrim]

Since there are no objections, I'm setting this to positive review.