First stab at a setup that uses Swiftly

Author: fwal

src/swiftly/index.ts

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+export * from "./swiftly";
+export * from "./install-linux";

src/swiftly/install-linux.ts

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+import { addPath, debug, info } from "@actions/core";
+import { downloadTool, find, extractTar, cacheDir } from "@actions/tool-cache";
+import { verify } from "../core/gpg";
+
+/**
+ * Setup Swiftly on Linux
+ */
+export async function setupLinux() {
+  let path = find("swiftly", "1.0.0");
+
+  if (!path) {
+    path = await download();
+  } else {
+    debug("Found cached Swiftly");
+  }
+
+  addPath(path);
+}
+
+async function download() {
+  info("Downloading Swiftly");
+
+  const arch = process.arch;
+  const url = `https://download.swift.org/swiftly/linux/swiftly-1.0.0-${arch}.tar.gz`;
+
+  debug(`Downloading Swiftly from ${url}`);
+
+  const [pkg, signature] = await Promise.all([
+    downloadTool(url),
+    downloadTool(`${url}.sig`),
+  ]);
+
+  await verify(signature, pkg);
+
+  const extracted = await extractTar(pkg);
+  debug(`Extracted Swiftly to ${extracted}`);
+
+  const cached = await cacheDir(extracted, "swiftly", "1.0.0");
+  debug(`Cached Swiftly to ${cached}`);
+
+  return cached;
+}

src/swiftly/swiftly.ts

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+import { info } from "@actions/core";
+import { cmd } from "../core";
+
+async function swiftly(...args: string[]) {
+  return await cmd("swiftly", args);
+}
+
+/**
+ * Install Swift using Swiftly
+ * @param version Version to install
+ */
+export async function installSwift(version: string) {
+  info("Initializing Swiftly");
+  await swiftly("init", "--skip-install", "--quiet-shell-followup");
+
+  info(`Installing Swift ${version}`);
+  await swiftly("install", "--use", version, "--quiet-shell-followup");
+}