Preprobe info for known crate type
Previously, we've calculated the set of crate types to learn about by
recursively walking the graph of units. However, to actually know
dependencies of a unit exactly, we must know target specific info, and
we don't know it at this moment (in fact, we are trying calculating it).
Note that crate-type calculation is already lazy, we don't have to calc
all crate-types upfront. So, let's just scrape this info once for
well-known crate types, and fill whatever is left lazily.
@alexcrichton would this approach work at all? I think it would, if `KNOWN_CRATE_TYPES` are all available for all target-tripples we support. Is it a valid assumption?
The larger picture is that I am trying to make unit dependency resolution eager and move it into the separate file. I even got something working, but I have to run dependency resolution three times, because it is not exactly idempotent for various reasons, including this target-info stuff :)
```
cx.prepare()?;
cx.build_unit_map(units.clone())?; // resolve dependencies
cx.probe_target_info(&units)?;
cx.build_unit_map(units.clone())?; // resolve again
cx.build_used_in_plugin_map(&units)?;
custom_build::build_map(&mut cx, &units)?;
cx.build_unit_map(units.clone())?; // and resolve one final time :)
```
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