add the ability to exclude some files from the package

[jaydp17]

What did you implement:

Closes #470

It allows the user to include only a subset of files in the package.

For example, it's very handy to exclude sourcemaps from the package.

How did you implement it:

# serverless.yml
custom:
  webpack:
    builtAssetIncludeGlob: "**/*.{js,json}"

☝️this is how we use it.

We were already using glob package and passing ** to include all files, this PR just makes that input to glob package as a configurable value from serverless.yml.

If not set in serverless.yml it defaults to **, including all files.

How can we verify it:

1. As described above, just add the option in serverless.yml.
2. Run sls package
3. Extract the zip file in .serverless and verify that .js.map files are not included in the package

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

[designfrontier]

@jaydp17 do you mind correcting the merge conflicts outlined above? We're working our way through the backlog of pull requests. Thanks!

[hassankhan]

Is this PR potentially a duplicate of #471?

Should we perhaps rename the config option to be includeFiles, so as to be consistent with #471?

/cc @designfrontier @HyperBrain @jaydp17

[HyperBrain]

Can it be that this functionality is quite dangerous? Currently webpack completely controls what is packaged, i.e. what is used is packaged. Interfering with that might render broken deployments, e.g. if you use the json-loader (which you should when using json files) and forget to include the json files.

[jaydp17]

@HyperBrain this won't interfere with webpack at all, as this runs after webpack has finished building.
After webpack has outputted whatever assets it wanted to (i.e. bundle.js, bundle.js.map, etc.), this glob is applied to those files.

@designfrontier give me some time, I'll resolve the merge conflicts.

@hassankhan, includeFiles feels ambiguous, without reading the docs and just by looking at the config, you can't be sure which files does it mean.
Thus I tried to be a little descriptive with builtAssetIncludeGlob key, because we want to let the user know that this glob applies on the asset files outputted by webpack.

However, I'm open to suggestions, if builtAssetIncludeGlob doesn't feel right.

[designfrontier]

I agree that includeFiled feels ambiguous and this makes sense as a way to trim down the size of the built lambda.

builtAssetIncludeGlob sounds clunky but also seems reasonable. I can't think of anything better... but I am not great at naming things sometimes.

[j0k3r]

We do have an option excludeRegex which remove some fiels from the zip.
https://github.com/serverless-heaven/serverless-webpack#exclude-files-with-regular-expression

Can this solve your problem?