directory
This cleans up the `//toolchain/install/BUILD` file and the tree
generally to be more focused on arranging the actual installation rather
than preparing inputs to that installation.
I picke `//toolchain/runtimes` so we can put other runtimes preparation
logic there, but open to any other suggested organization.
There are other runtimes things that would in theory make sense to move
such as the `prebuilt_runtimes` logic, but a subsequent PR will delete
those and so I'm leaving them where they are for now.
Chandler Carruth
86d05b598b