[feature] [2.0] predefined layouts when playing locally

[lasote]

• When you work, let's say with CMake, with conan local commands and/or building locally with the build system, if you have a source() method to retrieve the external sources, (e.g conan-center) you may want to adjust the layout as:

    def layout(self):
        cmake_layout(self)
        self.folders.source = "src"

The self.folders.source = "src" is because you don't want to end up with a bunch of cloned/unzipped files at the root directory of your package.

• It happens also with any other build system, for example, using Autotools (we don't have any preset layout for that) you might want to declare something like:

    def layout(self):
         self.folders.build = os.path.join("build", str(self.settings.build_type))
         self.folders.generators = os.path.join(self.folders.build, "conan")
         self.folders.source = "src"

Because you don't want either the cloned "src" or the build/generators littering your package folder.

• Given that the previous definition is totally random, not attached to Autotools at all, we should consider to add a basic_layout() helper.
• Given that in both cases we are interested in not littering the repo with the cloned sources, consider an argument to all the layouts external_sources=True to assign a "src" folder.
• Check if the Meson layout is generic and it could be renamed.

EDIT: Consider not having the "build_type" setting for a basic layout.