Reduce memory usage by releasing webpack stats objects after compile

[janicduplessis]

What did you implement:

The current implementation keeps the full webpack stats object for all functions. This object is huge and causes very large memory usage when using package: individually and a large number of functions.

The webpack stats object is used for 2 things, outputPath and external modules. Instead of keeping the full stats object around, we calculate the outputPath and external modules after compilation and discard the stats object. Then packageModules and packageExternalModules can use these values directly instead of the stats object.

How did you implement it:

Set outputPath and externalModules in the compile phase and discard webpack stats object.

How can we verify it:

Tested this change in a large serverless app.

Before: Memory usage slowly increases up to 7.2gb, builds in ~1500s
After: Memory usage hovers around 1.5gb, builds in ~1000s

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

[janicduplessis]

Accomplish something similar to #570, but simpler.

Should fix #299

[j0k3r]

This looks interesting, I'll try out on some project and let you know!
In the mean time, could you rebase against the master? Thanks 🙏

[janicduplessis]

Rebased!

[j0k3r]

Looks good to me 👍