FR: path namespaces.

[vic]

Currently namespaces are installed like this:

imports = [ (inputs.den.namespace "foo" sources) ]

that creates the alias option for foo at top-level and also a _modules.args.foo to access the namepace.

it would be cool to install

imports = [ (inputs.den.namespace ["bar" "foo"] sources) ]

that will create the top-level alias bar.foo for writing and _modules.args.bar.foo for reading.

this is because in denful we will likely have deeply nested namespaces and users might want to mount only some of them by path and not everything below bar, like bar.baz.