Passes the location from the root at which packages are imported on to
all packages.
The path is passed in as a parameter called 'locatedAt' which contains
a list of strings with each individual path component.
For example, the blog source in `services/tazblog` will have a list
with `[ "services" "tazblog" ]` passed in as the `locatedAt`
attribute.
This can be used for enabling features such as path-specific imports
when using things like buildGo.
Broadly speaking, the following things are included:
* there is now a uniform `args` struct that is passed to all
derivations, package headers have been changed appropriately
* overrides are now loaded from a separate `override` folder just
using read-tree.nix
* third-party packages have moved into the `third_party` attribute set
