hadrian: Rebuilding just one library

In #20775 (closed), I was made aware that we can rm -rf _build/stage1 and hadrian will happily rebuild everything with the stage1 compiler.

But it appears that it doesn't work if I remove a specific library: rm -rf _build/stage1/libraries/base will reconfigure base and then fail with

Error, file does not exist and no rule available:
  _build/stage1/libraries/base/build/GHC/Stack/CCS.hs-boot