Enable support for Node & Yarn installation

[pivotal-david-osullivan]

Summary

This PR adds support for detection of a package.json or yarn.lock, which will cause the following buildpacks/dependencies to be required as part of the build:

• yarn.lock - requires install of yarn and node dependencies (can be provided by the yarn & node-engine buildpacks)
• package.json - requires install of node dependency (can be provided by the node-engine buildpack)

The Javascript package manager file should be specified with the environment variable BP_JS_PACKAGE_MANAGER_FILE, which expects a path relative to your application root and points to a package.json or yarn.lock file. This variable defaults to "", therefore requiring explicit opt-in to this feature.

This will require both the Yarn and NodeEngine buildpacks to be added to the Java composite order group as optional.

Use Cases

Enables projects which build frontend/javascript code to run commands such as 'yarn' to build alongside Java code. See paketo-buildpacks/java#651.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[dmikusa]

Going with #216