Add package to require errors
Fix overlays + compilers and redirects

These features previously did not compose.  Now, compilers and redirects
are captured from the configuration after overlays have been applied.
Fix for setimmediate package

The setimmediate package has a setImmediate.js main module, so we have
to take care not to throw a warning about inconsistent module
identifiers.
Normalize redirects

This has the happy side-benefit of increasing support for packages
designed for Browserify, since the `browser` field gets converted to one
or more `redirects`.