Custom methods must be protected

[uilianries]

• Any method declared as public, must be provided from ConanFile
• Any custom method must be declared as protected
• Any private method is not be accepted

closes #126

Wiki:

#KB-H036: "CUSTOM METHODS"

When a new method have to be added to the recipe, and it is not a default one provided by ConanFile, it must added as protected:

class Recipe(ConanFile):
    def configure(self): # default methods are public
        pass

    def _configure_cmake(self): # custom methods are protected
        pass

    def __private_routine(self): # private should not be used. Use protected instead.
	    pass

[SSE4]

I don't get it, you query valid attrs and current attrs, why query invalid attrs with different method?
isn't result the same as current_attrs - valid_attrs?

[uilianries]

private methods are not filtered by dir method. e.g. def __foo(self):

[SSE4]

might be too strict, as we may define some magic methods (__str__, __eq__, __new__, __init__, __enter__, __dir__, etc)?

[uilianries]

but it's the exception you know. It sounds like cccl and other few recipes. We know that is not a regular case, but new contributors can use private instead of protected when writing a recipe, for instance.

[SSE4]

@uilianries also, I wonder, why do we forbid to use private members? is it somehow bad/harmful?

[uilianries]

@SSE4 private attributes are not bad, but how many times we needed them for a recipe? Maybe twice all this time, only for workarounds and helpers, which are not the regular flow.