feat: export per-component selectors

[bmv437]

Description

PR #4750 added per-component export entries for the DOM test-utils wrappers. This adds the
matching entries for the selectors wrappers.

Motivation: consumers who re-export or extend a subset of Cloudscape components need to import
individual wrappers so they can assemble their own ElementWrapper. Today the only selectors
entry point is the ./test-utils/selectors barrel, and importing the barrel adds the finders for
every component to the shared ElementWrapper. There is no way to narrow that down or remove
components from it. With per-component entry points, consumers can import just the wrappers they
want. The DOM side already supports this, but selectors did not.

Related links, issue #, if available: n/a

How has this been tested?

Added a unit test for getComponentsExports in build-tools/tasks/__tests__/package-json.test.js

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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