The shim_et/ subtree helps the Meta-internal buck2 build system also work in the
open-source repo.
Shims are how open-source buck2 supports a line like:
load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
In the open-source repo, fbsource//xplat (a Meta-internal root) doesn't exist.
The fbsource = shim_et line in ../.buckconfig tells buck2 to look in
shim_et/xplat/executorch/build/runtime_wrapper.bzl
instead.
NOTE: tools is a symlink to ../shim/tools because fbsource//
must refer here, but fbsource//tools in particular lives in shim/.