Remove deprecated wildcard folder mapping (#23256)

* Remove deprecated folder mapping Node v16 deprecated the use of trailing "/" to define subpath folder mappings in the "exports" field of package.json. The recommendation is to explicitly list all our exports. We already do that for all our public modules. I believe the only reason we have a wildcard pattern is because our package.json files are also used at build time (by Rollup) to resolve internal source modules that don't appear in the final npm artifact. Changing trailing "/" to "/*" fixes the warnings. See https://nodejs.org/api/packages.html#subpath-patterns for more info. Since the wildcard pattern only exists so our build script has access to internal at build time, I've scoped the wildcard to "/src/*". Because our public modules are located outside the "src" directory, this means deep imports of our modules will no longer work: only packages that are listed in the "exports" field. The only two affected packages are react-dom and react. We need to be sure that all our public modules are still reachable. I audited the exports by comparing the entries to the "files" field in package.json, which represents a complete list of the files that are included in the final release artifact. At some point, we should add an e2e packaging test to prevent regressions; for now, we should have decent coverage because in CI we run our Jest test suite against the release artifacts. * Remove umd from exports Our expectation is that if you're using the UMD builds, you're not loading them through a normal module system like require or import. Instead you're probably copying the files directly or loading them from a CDN like unpkg.