Bug 1287023 - Allow to use delayed_getattr in more cases. r=chmanchester

Until now, it's not been possible to do something as straightforward as:

option('--foo', default=delayed_getattr(milestone, 'is_nightly'))

The reason is that option's default needs what it's given, if it's a
@depends function, to depend on --help.

But we can't have every delayed_getattr add dependencies on --help,
because that would make unwanted things to depend on --help and run
when displaying the help.

Until we can totally remove --help dependencies, this change makes the
resulting @depends function created by delayed_getattr depend on --help
if the @depends function it's given already depends on --help.

build/moz.configure/util.configure

@@ -137,14 +137,21 @@ def namespace(**kwargs):
 #       return namespace(foo=value)
 #   set_config('FOO', delayed_getattr(option, 'foo')
 @template
+@imports('__sandbox__')
 def delayed_getattr(func, key):
-    @depends(func)
-    def result(value):
+    _, deps = __sandbox__._depends.get(func, (None, ()))
+
+    def result(value, _=None):
         # The @depends function we're being passed may have returned
         # None, or an object that simply doesn't have the wanted key.
         # In that case, just return None.
         return getattr(value, key, None)
-    return result
+
+    # Automatically add a dependency on --help when the given @depends
+    # function itself depends on --help.
+    if __sandbox__._help_option in deps:
+        return depends(func, '--help')(result)
+    return depends(func)(result)
 
 
 # Like @depends, but the decorated function is only called if one of the