Introduce a package for installation management.

[chandlerc]

This package contains the BUILD logic for creating an installable tree of data files and executables for the toolchain, and a library to facilitate toolchain code in accessing their data files within this installation.

Currently, this only really manages access to the LLVM binaries installed and used by the Clang runner for linking, but eventually other data files like the prelude and runtime libraries will be fleshed out as well.

This also provides a natural place for us to build out packaging rules to create installable packages in various formats, but that remains future work.

Depends on both #3989 and #3992 -- only the last commit should be reviewed here.