devmode: migrate into pkgs/by-name

[alejandrosame]

This move will also make the tool available outside of nixpkgs codebase.

[chayleaf]

perhaps use overrideAttrs here?

[fricklerhandwerk]

Looks good in general, no nitpicks from me today.

[mightyiam]

For one, I never intended it to be public. I didn't want to lock the API.

[infinisil]

I'd agree with @mightyiam here, let's not expose this in pkgs unless we're certain of its interface. Instead third-party users can still callPackage the path to the package directly (which might break in the future as we should be expecting, generally paths are not a stable interface).