Autodetect modules to instrument

Currently for a module to be automatically instrumented by this agent, there needs to be both a js file with the same name as the module under `lib/instrumentation/modules` and the name of the modules needs to be added to the [`MODULES`](https://github.com/elastic/apm-agent-nodejs/blob/dd20f63cfc56f3078c5e24e56a0f58a2d859a999/lib/instrumentation/index.js#L16) array.

Likewise, if the tests for the instrumentation are placed in a subdirectory inside `test/instrumentation/modules` that directory needs to be added to the [`directories`](https://github.com/elastic/apm-agent-nodejs/blob/dd20f63cfc56f3078c5e24e56a0f58a2d859a999/test/test.js#L65-L76) array.

It would make sense to automate this, so that these files and directories were just automatically detected when requiring the module and running the tests.

For the tests, I don't think we can get rid of the `directories` array completely as we'd like to ignore certain directories, but if we could add glob support to the entries in the array that would solve it.