This reverts commit
637033c410.
What wasn't clear originally is that we require that required packages
have their `package.json` resolvable. So either they don't have an
`exports` field, or their `package.json` is listed in the `exports`
field. The React beta types didn't have `package.json` entry in
`exports` so we accidentally thought they weren't installed.
I published [`19.0.0-beta.2` that has `package.json` in
`exports`](13879ee571 (diff-81cc573aa0c2bd0e13f9463499747741704aabccd7474f544db710befd7bcfc4R44))
so we can restore the old behavior. It's still questionable IMO to do
all that I/O work just for a nicer error message that may contain
false-positive but that's for another time.
41a80533f9