Split into modules and reorganize

src/FetchOneCommand.ts

@@ -0,0 +1,52 @@
+import { Command } from "clipanion";
+
+import {
+  Cache,
+  CommandContext,
+  Configuration,
+  LocatorHash,
+  Project,
+  StreamReport,
+} from "@yarnpkg/core";
+
+// Internal command that fetches a single locator.
+// Used from within Nix to build the cache for the project.
+export default class FetchOneCommand extends Command<CommandContext> {
+  @Command.String()
+  locatorHash: string = ``;
+
+  @Command.Path(`nixify`, `fetch-one`)
+  async execute() {
+    const configuration = await Configuration.find(
+      this.context.cwd,
+      this.context.plugins
+    );
+    const { project } = await Project.find(configuration, this.context.cwd);
+    const cache = await Cache.find(configuration);
+
+    const fetcher = configuration.makeFetcher();
+
+    const report = await StreamReport.start(
+      { configuration, stdout: this.context.stdout },
+      async (report) => {
+        const pkg = project.originalPackages.get(
+          this.locatorHash as LocatorHash
+        );
+        if (!pkg) {
+          report.reportError(0, `Invalid locator hash`);
+          return;
+        }
+
+        await fetcher.fetch(pkg, {
+          checksums: project.storedChecksums,
+          project,
+          cache,
+          fetcher,
+          report,
+        });
+      }
+    );
+
+    return report.exitCode();
+  }
+}