Do not swallow errors in case npm ls fails without an empty stdout and use spawn for child processes

[HyperBrain]

What did you implement:

If npm ls fails the plugin tries to analyze stderr for npm errors that can be ignored. If none are found, it will pass stdout to JSON.parse().
The case that any non-npm error was happening was not handled correctly (e.g. if the stdout maxbuffer size got exceeded by a very big dependency section). The plugin passed the empty stdout to JSON parse and failed there without telling the exact reason for the failure.
Additionally, Node's buffered stdio streams do not work reliably and the exec method is not meant to be used for processes that might return data of arbitrary length.

How did you implement it:

The packager executables are now executed with child_process.spawn which allows for stdio streams of arbitrary length. On execution failure, a proper SpawnError is now thrown that contains the full stdout and stderr streams.
Additionally this eliminates the need of the packExternalModulesMaxBuffer buffer configuration which has been removed from the README.
The implementation should seamlessly work as before.

How can we verify it:

Use a package.json with many dependencies, so that the output of npm ls exceeds the max stdout buffer default of 200k with the old serverless-webpack version.
It should work with this branch now.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[HyperBrain]

Hi @franciscocpg . Could you do me a favor and test if the current master still works properly for you? I switched the packager invocations to use child_process.spawn instead of exec here, but only verified Windows yet. It would be great if I had an additional success report before I release the fix 😃 . Thanks.

[franciscocpg]

hi @HyperBrain
I'll do it.

A sls package would be enough?

[HyperBrain]

Thanks 😏 . Yes I think so, because it runs through the whole npm or yarn processes. There's no need to actually deploy it.

[franciscocpg]

@HyperBrain
It works seamlessly in one of our biggest project with npm process.

I'm unable to run using yarn. See #370 (comment).

[HyperBrain]

Thanks for the tests 👍 . I commented on the issue with yarn. Maybe the plugin could emit a better error message for that case.

[franciscocpg]

Was able to test with yarn using the workaround I've mentioned in #370 (comment). It worked seamlessly too.