Add a ListInstalled function

I've needed at times to list the packages that are currently listed on
a system. Search("") is equivalent to that, but since we have List()
which is equivalent to Search("*"), I figured this might be handy to
have as well.

Author: mickael-carl

apt.go

@@ -44,6 +44,11 @@ func List() ([]*Package, error) {
 	return Search("*")
 }
 
+// ListInstalled returns a list of packages installed on the system.
+func ListInstalled() ([]*Package, error) {
+	return Search("")
+}
+
 // Search list packages available in the system that match the search
 // pattern
 func Search(pattern string) ([]*Package, error) {