Protect directories from source removal

[fg-j]

Describe the Enhancement

There should be a way to tell the buildpack not to delete certain parts of the source code in /workspace.

Possible Solution

Respect the environment variables that https://github.com/paketo-buildpacks/source-removal respects so that users can protect files from being removed.

Motivation

I was trying to build https://github.com/Squidex/squidex with the buildpack. The app has a frontend and backend that are designed to run in the same container. The Dockerfile multistage build transpiles the JavaScript frontend into static assets in a directory, and compiles the .NET backend into *.dlls.

I can't build this app with buildpacks because the dotnet-publish buildpack removes everything in the working directory besides the compiled app. My static assets get deleted if I build them before this buildpack runs, and the JavaScript source code gets deleted if I try to transpile it after this buildpack runs.