refactor: move loco-rs crate under loco-rs folder

[joshka]

this cleans up the root and makes it easier to focus on just one crate at a time

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This is a PR that's sort of a RFC - I won't be offended if you close it as not interested.
It may be related some ideas in little to #881

[jondot]

Thanks! this is a nice take actually.
We're going to dedicate 0.12.0 under the theme of "removing, simplifying" (#881)

The idea is that when the project started I've put everything bunched up, so that we can see and in that regards "feel" our deps, it makes rust-analyzer's work a bit heavier, it makes the target folder a big bigger, and it makes shipping a release a bit slower.
So we're now putting things into their natural places to simplify and make things more lightweight and easier.

For example: the macros project, did not take off (we thought giving everything a macro for better DX, but that turned out to be un-Rustful)
The extras project evolve in a different speed, and is more "static", and so should be out of the workspace, but closely tested against Loco versions
And @kaplanelad is now experimenting with unifying loco-cli and loco into a single binary+lib crate, so that we can couple new project generation with its testing, and with current versions of the framework (just like Rails does).

We'll see how the simplification effort goes and definitely want to look at this PR again.