feat(rollup): simplify external (#80)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**: basically this: Unless you're bundling a `umd` file, all files in `node_modules` should be marked as external. That's what this code does.

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**Why**: I noticed that when I upgraded to emotion 10 in one of my packages and started using the babel plugin it swapped `@emotion/styled` for `@emotion/styled-base` which is not marked as an external with this logic and it kinda messed things up. So I improved the external to be what I think is intended.

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**How**: the predicate function is called with the same module multiple times which is what made this more tricky than I thought. First it's called with the module id as it appears in the code, then it's called with the full path to the module.

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I'd love some input from everyone who's touched the rollup config here. @Andarist, @alexandernanberg, @eliperkins, and @fobbyal. I think this is good, but it'd be nice to get a sanity check. Thanks!

Author: Kent C. Dodds

src/config/rollup.config.js

@@ -88,8 +88,19 @@ if (isPreact) {
 }
 
 const externalPattern = new RegExp(`^(${external.join('|')})($|/)`)
-const externalPredicate =
-  external.length === 0 ? () => false : id => externalPattern.test(id)
+
+function externalPredicate(id) {
+  const isDep = externalPattern.test(id)
+  if (umd) {
+    // for UMD, we want to bundle all non-peer deps
+    return isDep
+  }
+  // for esm/cjs we want to make all node_modules external
+  // TODO: support bundledDependencies if someone needs it ever...
+  const isNodeModule = id.includes('node_modules')
+  const isRelative = id.startsWith('.')
+  return isDep || (!isRelative && !path.isAbsolute(id)) || isNodeModule
+}
 
 const filename = [
   pkg.name,