Amend manifest search to work well with multi-projects builds

In my multi-project build, replicant would incorrectly set the package name of one of the sub-projects (which is a library project). I amended the implementation so that it looks for the manifest in the current directory and all sub-directories and checks if the sub-project is an app project. 

I couldn't really figure out how to do a couple of things properly, so suggestions are welcome:

1. if the current project is a library project, the package will be set to the one of the sample app, which is probably not what you want. But how can replicant know that's the case? A possibility would be to look for know app directory names like "sample", skip those and fall back to the library project.

2. how to write tests for this, on the other hand there weren't any for the previous implementation.

just a style thing, but the `?` in Ruby already denotes that this is a boolean property. Drop `is_`