chore(papi): keep the upstream sources in upstream-src/

The pezkuwi-dev build treats any package with a src/ directory as its
own: it requires our license header on every file and builds it. These
are upstream files that must stay byte-identical to the source embedded
in dist/, so they live in upstream-src/ instead, which neither the build
nor the root tsconfig picks up.

packages/papi-utils/upstream-src/AbortError.ts

@@ -0,0 +1,6 @@
+export class AbortError extends Error {
+  constructor() {
+    super("Abort Error")
+    this.name = "AbortError"
+  }
+}