Package exports browser field interaction

[guybedford]

I thought I followed something here previously where esbuild had to deal with the interaction between the exports field resultant target path itself being a key in the "browser" field.

There is an example repo for this applying to solid-js in https://github.com/patdx/package-exports-test by @patdx.

In discussions with @sokra I recall Webpack wanting this behaviour, so we implemented it in RollupJS as well to match, but I was under the assumption that we were matching esbuild, although it appears that is not the case.

We can adjust RollupJS to match either pattern, was hoping we could get some further discussion going on which way to move here @evanw.