feat: SwiftWriter imports types that are written to code

[jbelkins]

Issue #

awslabs/aws-sdk-swift#1533

Description of changes

• SwiftWriter will now automatically import any symbol that is written to code using the $N, $D, or $T format specifier.
• Swift symbols may now have their declaration type added to them; if they do, then only that type is imported. If not, the entire enclosing module is imported instead.
• ClientRuntime.Core Swift symbols have their Swift declaration type added so they may be imported. Other symbols will be modified in future PRs (and existing SwiftWriter.import() calls deleted) in future PRs.

also:

• WeatherSDK updated with new imports

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.