Deprecate the asdf.asdftypes module in favor of asdf.types

[drdavella]

This PR also contains some additional code cleanup: AsdfTypeIndex has moved to a separate module.

[stsci-bot]

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[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

If there are any issues with this message, please report them here.

[coveralls]

Coverage increased (+0.004%) to 94.769% when pulling fbd8256 on drdavella:refactor-asdftypes into 96058b8 on spacetelescope:master.