Generate server->shared cross realm links

[magnalite]

We want to reduce the number of packages we have in the datamodel, primarily this means keeping only one copy of each package. Some packages in the server realm will depend on shared packages, however due to this optimisation they will be held in separate locations in the datamodel.

To allow server packages to require shared packages we can generate links which reference across realms. This is notable because we cannot generate these links as relative paths. We also don't know the absolute location of where shared packages are as Rojo manages their location.

In order to know where the shared packages are to generate these cross realm links an additional field has been added to the wally.toml. As an example the wally.toml may look like this.

[package]
name = "magnalite/my-package"
version = "1.4.2"
license = "Apache-2.0"
registry = "https://github.com/UpliftGames/wally-index"
realm = "server"

[place]
shared-packages = "game.ReplicatedStorage.Packages"

[dependencies]
Promise = "evaera/promise@3.1.0"

I see it fit to put this in the "place" table to emphasise this is metadata describing the place it will be sync'd into and not information related to the package itself. This can also be where we add any further metadata about where the package will be sync'd we may require.