Switch to ESM for source and test, use CommonJS for dist (#806)

scripts/build-dist.mjs

@@ -0,0 +1,33 @@
+import { rm } from "node:fs/promises";
+import { build } from "esbuild";
+
+const builds = [
+  {
+    entryPoints: ["src/setup-uv.ts"],
+    outfile: "dist/setup/index.cjs",
+    staleOutfiles: ["dist/setup/index.mjs"],
+  },
+  {
+    entryPoints: ["src/save-cache.ts"],
+    outfile: "dist/save-cache/index.cjs",
+    staleOutfiles: ["dist/save-cache/index.mjs"],
+  },
+  {
+    entryPoints: ["src/update-known-checksums.ts"],
+    outfile: "dist/update-known-checksums/index.cjs",
+    staleOutfiles: ["dist/update-known-checksums/index.mjs"],
+  },
+];
+
+for (const { staleOutfiles, ...options } of builds) {
+  await Promise.all(
+    staleOutfiles.map((outfile) => rm(outfile, { force: true })),
+  );
+  await build({
+    bundle: true,
+    format: "cjs",
+    platform: "node",
+    target: "node24",
+    ...options,
+  });
+}