drop requirement for setuptools as runtime dependency

[boegel]

setuptools was only required because of the pkg_resources.declare_namespace statements that were used to declare the various easybuild.* (sub)namespaces.

There's a replacement in the Python standard library however, being __import__('pkgutil').extend_path, and it's even the recommend approach (see https://packaging.python.org/guides/packaging-namespace-packages/#pkgutil-style-namespace-packages).

One side-effect is that the __init__.py files should only contain the extend_path statement, nothing else. Because of this, some things had to be moved, i.e.:

• the DUMMY_TOOLCHAIN_NAME and DUMMY_TOOLCHAIN_VERSION constants were moved from easybuild.tools.toolchain to easybuild.tools.toolchain.toolchain
• the DEVEL_MODULE_SUFFIX constant was moved from easybuild.tools.module_naming_scheme to easybuild.tools.module_naming_scheme.mns
• the ModuleNamingScheme class is no longer available from the easybuild.tools.module_naming_scheme namespace, only from the easybuild.tools.module_naming_scheme.mns namespace (cfr. changes in support for hierarchical module naming schemes #953)

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @boegel!