find all global packages, list them by directory
one module to rule them all, one module to find them, one module to bring them all
and in the darkness, lexically bind them
find all globally installed packages, resolving to a list of directories representing global packages.
var findGlobal =
Takes a single node-style callback, resolves to either an error or a list
of directories. Directories are vetted by the presence of a file called
package.json (but not its contents).