interpreter: Add path() method to ExternalProgramHolder types

In practice, this means we can do

 flex = find_program('flex')
 flex.path() # Gives the full path to the flex binary

mesonbuild/interpreter.py

@@ -242,11 +242,15 @@ class ExternalProgramHolder(InterpreterObject):
     def __init__(self, ep):
         InterpreterObject.__init__(self)
         self.held_object = ep
-        self.methods.update({'found': self.found_method})
+        self.methods.update({'found': self.found_method,
+                             'path': self.path_method})
 
     def found_method(self, args, kwargs):
         return self.found()
 
+    def path_method(self, args, kwargs):
+        return self.get_command()
+
     def found(self):
         return self.held_object.found()