Enable support for node & yarn detection

[pivotal-david-osullivan]

Enable support for node & yarn detection

Alternative to #215

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)

To enable support for detection of these files, users should set the environment variable BP_JAVA_INSTALL_NODE to true (defaults to false to prevent false-positive node builds).

The buildpack will check the application root for the package.json or yarn.lock file. Users can set variables supported by the NodeEngine buildpack, such as BP_NODE_PROJECT_PATH, which can contain a sub-directory within the app root to search for these files.

Full implementation 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).

[TisVictress]

Bump 🙂
I'm happy to help or answer questions regarding node