Add ability to specify the node_modules relative dir

[ypxing]

What did you implement:

serverless-webpack uses node_modules to find out all peerDependencies but the assumption is that node_modules is in the same directory as package.json. But this is not always true specially for monorepo case where node_modules are usually in one parent directory.

This PR is to remove this assumption by adding one customer configuration nodeModulesRelativeDir. By default it follows current behaviour.

It can addresses issue #431

How did you implement it:

How can we verify it:

Testing cases has been updated to cover with/without the new configuration.
README has been updated.

Testing shows below error message is gone and peerDependencies can be packed properly

WARNING: Could not check for peer dependencies

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

[j0k3r]

Can't we have an error or a warning when nodeModulesRelativeDir is not a real directory? Like for a misconfiguration?

[j0k3r]

Can you rebase against the master and fix conflicts? Thanks 🙏

[ypxing]

Sorry for late response. Rebase is done