Introduce mojo_edk_source_set and use it in //mojo/edk

This CL is a step toward making the //mojo/edk buildfiles flexible wrt the
location of the Mojo EDK in a client repo. We introduce
//mojo/edk/mojo_edk.gni, which contains a mojo_edk_source_set template. A
mojo_edk_source_set is just a mojo_sdk_source_set that (a) doesn't restrict its
external dependencies and (b) has additional variables to allow for e.g.
dependencies specified relative to the Mojo EDK.

Note that we require that the Mojo EDK be a sibling of the Mojo SDK in a client
repo, and rely on that fact. The addition of the extra mojo_edk_* variables in
mojo_edk_source_set are only for clarity when writing BUILD.gn files.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/759633004