refactor(workspaceutil): unify file collecting utilities

[Hweinstock]

Problem

collectFiles and collectFilesForIndex serve different purposes, but implement the same core functionality. This core functionality should be not duplicated in two places.

Solution

• Increase testing coverage for both methods.
• Refactor collectFiles to be general enough to handle collectFilesForIndex use case. This includes the ability to avoid reading the files when we are indexing.
• collectFilesForIndex now directly calls collectIndex then does its additional logic with the result.

Verification

• Tested @workspace in chat with some different prompts. Seems to behave the same as before.
• Tested /review on a project.

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• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Hweinstock]

/runIntegrationTests

[jpinkney-aws]

🎉