Allow resource-less libraries

[cboulanger]

Many libraries contain only code, and no resources in a directory specified by the provides.resource key of Manifest.json. If the default directory named "resource" is missing, the compiler emits a warning.

It should be possible remove the provides.resource key so that the warning does not show up in what is a perfectly legal resource-less library. ATM, this does not work, because both removing the key of leaving it empty will result in aborting the compilation process with an error.

[johnspackman]

A related issue is that the compiler requires that there is a Manifest.json in order to operate a compile.json - the Manifest belongs to a library and describes what is required to use that library as well as what code or resources are provided by that library.

At the moment, the package manager assumes that the current directory's Manifest is the root of all requirements, but really it should start at the first library and recurse through the library dependencies to find all libraries and resolve required packages.