refactor(connector): rename source crate to dml

[BugenZhao]

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What's changed and what's your intention?

• Move remaining modules related to connector sources from the source crate into the source::reader module under the connector crate.
• The source crate only contains the DML-related modules (which was called TableSource long ago), so we can directly rename it to dml.

Also some minor dead code and renaming refactors.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.