When building plugins externally with `zeek-config` (as opposed to using
`ZEEK_DIST`), they point into the install prefix. There, we add a new
file `ZeekPluginBootstrap.cmake` that helps `ZeekPlugin.cmake` to find
everything else it needs from there.
Our template for plugins generates a `configure` script that sets
various variables with values from `zeek-config`. We only need
`BROKER_ROOT_DIR` with the new bootstrapping logic. Everything else, we
can get from the new bootstrapping file and from the CMake package file
for Zeek.
