[instrumentation] replace `cpx2` with a local module in `tests/fixtures/` exclusively for testing

[pichlermarc]

In @opentelemetry/instrumentation we test RequireInTheMiddleSingleton by using the cpx2 package. https://github.com/open-telemetry/opentelemetry-js/blob/63d74cdc366fc337be66c1a766a2cc8e5275a85a/experimental/packages/opentelemetry-instrumentation/test/node/RequireInTheMiddleSingleton.test.ts#L42-L43

As we don't use the cpx2 package anywhere else anymore, having a local test package would help us get rid of that dependency

To complete this issue we need to:

• [ ] add a local package that mimics how cpx2 is structured for testing in ./tests/fixtures/
• [ ] update the tests to use that package instead
• [ ] remove the 'cpx2' devDependency from package.json

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It looks like cpx2 is only used as a test package for RequireInTheMiddleSingleton, so picking some other module, or manually creating a local module in a fixtures/ subdir and using that directly would work as well. However, using cpx2 seems fine.

Originally posted by @trentm in https://github.com/open-telemetry/opentelemetry-js/pull/4510#pullrequestreview-1920997060

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