ref: delete unreachable upstream-fetcher modules

Stacked on the modifications PR. Now that no live code path references
the per-package fetchers, the shared HTTP/parsing helpers, the
in-process normalizer, or the example template, delete them. Pure
deletion — no behavior change.

- ~93 per-package <pkg>/releases.js fetcher modules.
- _common/{brew,fetcher,git-tag,gitea,github,github-source,
  githubish,githubish-source}.js shared HTTP/parsing helpers.
- _webi/normalize.js in-process normalization layer (cache files
  arrive normalized from webicached).
- _example/releases.js fetcher template for new packages.

The Go cache daemon (webicached) is now the sole producer of release
metadata; the Node process never makes an upstream request.

vim-commentary/releases.js

@@ -1,19 +0,0 @@
-'use strict';
-
-var git = require('../_common/git-tag.js');
-var gitUrl = 'https://github.com/tpope/vim-commentary.git';
-
-module.exports = async function () {
-  let all = await git(gitUrl);
-
-  return all;
-};
-
-if (module === require.main) {
-  module.exports().then(function (all) {
-    all = require('../_webi/normalize.js')(all);
-
-    let samples = JSON.stringify(all, null, 2);
-    console.info(samples);
-  });
-}