Add ?module support for HTML files

Closes #113

modules/actions/serveHTMLModule.js

@@ -0,0 +1,50 @@
+const etag = require("etag");
+const cheerio = require("cheerio");
+
+const getContentTypeHeader = require("../utils/getContentTypeHeader");
+const rewriteBareModuleIdentifiers = require("../utils/rewriteBareModuleIdentifiers");
+
+function serveHTMLModule(req, res) {
+  try {
+    const $ = cheerio.load(req.entry.content.toString("utf8"));
+
+    $("script[type=module]").each((index, element) => {
+      $(element).html(
+        rewriteBareModuleIdentifiers($(element).html(), req.packageConfig)
+      );
+    });
+
+    const code = $.html();
+
+    res
+      .set({
+        "Content-Length": Buffer.byteLength(code),
+        "Content-Type": getContentTypeHeader(req.entry.contentType),
+        "Cache-Control": "public, max-age=31536000, immutable", // 1 year
+        ETag: etag(code),
+        "Cache-Tag": "file, html-file, html-module"
+      })
+      .send(code);
+  } catch (error) {
+    console.error(error);
+
+    const errorName = error.constructor.name;
+    const errorMessage = error.message.replace(
+      /^.*?\/unpkg-.+?\//,
+      `/${req.packageSpec}/`
+    );
+    const codeFrame = error.codeFrame;
+    const debugInfo = `${errorName}: ${errorMessage}\n\n${codeFrame}`;
+
+    res
+      .status(500)
+      .type("text")
+      .send(
+        `Cannot generate module for ${req.packageSpec}${
+          req.filename
+        }\n\n${debugInfo}`
+      );
+  }
+}
+
+module.exports = serveHTMLModule;