Fetch Git submodules lazily

[edolstra]

Motivation

This isn't particularly useful right now, since we still copy the entire Git input to the store. But once we have lazy trees, this will avoid fetching submodules unless they're actually needed.

Note: submodules in workdirs are not lazy yet.

Context

• Blocked on Lazy paths #11367

Priorities and Process

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[roberth]

Looks alright overall, but can't be tested until we have at least

• Lazy paths #11367
  or some other solution similar to
• Lazy trees #6530