Normalize .less => .css generation

This is a split from MDL-39276

Could we also get this as an opportunity to "normalize" the way we are going to use .less and generated .css files?

In order to be able to "compute" (aka CI servers) all the less compilations to perform we would need something like:

1) All the main .less files (not the included ones) to stay always under theme/xxx/less
2) Each one of those main files will lead to, always, a .css file under theme/xxx/styles with the same name than the original (aka, moodle.less => moodle.css, editor.less => editor.css...).

Please consider it, else it's not easily guessable (right now we are generating a "generated.css" file instead of moodle.css).

Ciao