Add API to define a package finder and make the -package key resolver use defined finder and only fall back to pkg-config framework.

[ethouris]

There should be some method that allows to specify contents of the package and possibly how to search for it and how to autodetect it. The package database will be searched first for a package, and the pkg-config framework will be a default fallback.

The "package" should be treated like something being a "target that is always built, and if it's not, then there's nothing we can do". This will still have to be used through -packages key (not -depends), but in future some methods for automatically resolving lacking packages, should there be defined a method for that, can automatically install packages and make them available.

The command to define a package would be something like:

ag-define-package <name> -include <header dir> -lib <lib dir> -flags ...
ag-provide-package <name> <procname>