Improve error messaging for apps containing multiple projects

[fg-j]

Describe the Enhancement

It's a common pattern in .NET apps for one "app" to contain several projects. Like this test fixture. Often, when users first attempt to build these apps with buildpacks, they struggle to correctly use BP_DOTNET_PROJECT_PATH and set the build context for their build (e.g. with pack build myimage --path to/src). Users may mistakenly:

• not set BP_DOTNET_PROJECT_PATH, making it ambiguous which project they wish to build
• set BP_DOTNET_PROJECT_PATH to the wrong proj file
• limit the build context to only one project that contains references to other projects.

It would be a better user experience if the buildpack provides better advice to users about whether to adjust their build context and/or change the value of BP_DOTNET_PROJECT_PATH.

Possible Solution

Motivation