fix(config): warn instead of erroring when workspace member dir is missing (#34511)
Listing a workspace member by literal path required the directory to be
present on disk, otherwise workspace discovery failed with "Could not
find config file for workspace member". This broke the documented Docker
workflow that only copies the relevant member subset into the image, and
it diverged from npm, which silently tolerates missing workspace
members. Now we emit a warning and skip the missing member, so any
remaining members install and run normally.
Genuine misconfigurations are unchanged: pointing at a config file
instead of a directory still errors, and an npm workspace member whose
directory exists but contains no `package.json` still errors.
Fixes #28365