refactor: check assertions per import statement

[nayeemrmn]

Closes #132
Fixes #160
Supersedes #155

Assertions appropriately no longer influence module loading/parsing. Instead they are checked in a pass at the end; resulting errors are stored in Import::errors: Vec<ImportError>. 'Import errors' are a third kind of error alongside module slot errors and resolution errors which are now checked in ModuleEntryIterator::validate().

[dsherret]

I'll review the rest of it later.

[dsherret]

Thanks for this @nayeemrmn. It's looking better than main, but I'm not sure about the name "imports". Perhaps this should be something more generic because it also includes exports and typescript path references?

[nayeemrmn]

Regarding 2.0 breakages, seems like the only breakage is Dependency::assertionType. Could we keep that as legacy for a month to unblock this PR?

[dsherret]

Probably not because merging this would also require more work upgrading the rest of the code in other repos and that would block progress on getting npm specifiers in deno_graph, which is the priority at the moment because we want to get them in a more ideal state for integration with Deploy and other tooling. It's just waiting a couple weeks until early March.

[nayeemrmn]

@dsherret I went ahead and preserved assertionType in the serialization for legacy. It's a very self-contained, small addition in serialize_dependencies(). This can be considered for landing without breaking deno info now.

[nayeemrmn]

With import assertions becoming import attributes and them being part of the cache key, this PR goes in the wrong direction for the future. Closing.