fix(vpc): recognize Public subnets by Internet Gateway

[rix0rrr]

The VPC importer used to look at MapPublicIpOnLaunch as a shortcut
for determining whether a Subnet was public or private.

The correct way to do it is to look at the route table and see if
it includes an Internet Gateway.

Fixes #3706.

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Pull Request Checklist

• Testing

• Unit test added (prefer to add a new test rather than modify existing tests)
• CLI change? Re-run/add CLI integration tests

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  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
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  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
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  • Design: for significant features, follow design process