Add pa11y scanner

Previously, we did accessibility scanning for every file in every
push / build / PR, which took > 25 mins each run.

Now, we more carefully curate the files that need to be scanned, to
limit the scan time.

This commit:

- Adds `npm run pa11y:list-files` command which runs a site build and
  outputs the files to be scanned for the current changeset
- Adds a per-page pa11y scan, which lists pages that have changed or
  whose layouts have changed
- Adds a site-wide pa11y scan: if files that affect the whole site
  change, take a sample of files from across the site to check
- Incidentally deletes an unused layout
- Moves _plugins/ to lib/ and ignores lib/ in the 11ty build
- Pins the glob package to 8, because 9 and higher breaks something
  with the SVG Sprite package

The code reads a little weird, and one more refactor could help.

.eleventy.js

@@ -27,6 +27,7 @@ const { postsCollection, servicesCollection, tagsCollection } = require('./confi
 const { headingLinks } = require('./config/headingLinks');
 const { contrastRatio, humanReadableContrastRatio } = require('./config/wcagColorContrast');
 const privateLinks = require ('./config/privateLinksList');
+const { pa11yScan } = require('./lib/pa11y')
 
 const { imageShortcode, imageWithClassShortcode } = require('./config');
 
@@ -321,6 +322,8 @@ module.exports = function (config) { /* eslint-disable-line func-names */
     ghostMode: false,
   });
 
+  config.on('eleventy.after', pa11yScan);
+
   // Set image shortcodes
   config.addLiquidShortcode('image', imageShortcode);
   config.addLiquidShortcode('image_with_class', imageWithClassShortcode);

.eleventyignore

@@ -5,3 +5,4 @@ README.md
 config
 docs
 spec
+lib

lib/commit_differ.js

@@ -0,0 +1,11 @@
+const { Differ } = require('./differ');
+
+// Lists files that were modified in the last commit
+// Used for CI pa11y checks, to check the last commit
+class CommitDiffer extends Differ {
+  command = 'git diff-tree --no-commit-id --name-only -r $(git rev-parse HEAD)';
+}
+
+module.exports = {
+  CommitDiffer,
+}

lib/differ.js

@@ -0,0 +1,16 @@
+const {execSync} = require('child_process');
+
+// Lists files that have been modified since the last commit
+// Used in local development, to check accessibility prior to
+// committing changes
+class Differ {
+  command = 'git ls-files --modified';
+
+  changedFiles() {
+    return String(execSync(this.command)).split(/\n/).filter(x => x !== '')
+  }
+}
+
+module.exports = {
+  Differ,
+}

lib/document.js

@@ -0,0 +1,47 @@
+const {execSync} = require('child_process');
+
+// Takes an 11ty "result" (a post or page), the dir (folder/config),
+// and a list of changed files from the appropriate differ class.
+//
+// A Document knows whether or not it's in the list of changed files,
+// and Document#shouldScan returns true if it should be included in
+// the next pa11y check.
+class Document {
+  constructor(result, dir, changedFiles) {
+    this.result = result;
+    this.dir = dir;
+    this.layout = this.layoutName();
+    this.changedFiles = changedFiles;
+  }
+
+  shouldScan() {
+    return this.didSourceChange() || this.didLayoutChange()
+  }
+
+  // Returns whether the input (source) file was modified
+  // The slice(2) is to remove the "./" at the beginning of the path
+  didSourceChange() {
+    return this.changedFiles.includes(this.result.inputPath.slice(2))
+  }
+
+  // Returns whether this file's layout changed
+  didLayoutChange() {
+    return this.changedFiles.includes(`${this.dir.layouts}${this.layout}`)
+  }
+
+  // Gets the name of the layout for this file. If there's no layout,
+  // assume it uses the "default" layout.
+  // @todo Change this from system `grep` to parsing the YAML frontmatter
+  // @todo Assuming the "default" may not be a safe assumption
+  //
+  layoutName() {
+    let maybeMatch;
+    const layoutMatcher = `grep -i "^\s*layout: " "${this.result.inputPath}"` /* eslint-disable-line no-useless-escape */
+    try { maybeMatch = execSync(layoutMatcher) } catch { return 'default' }
+    return String(maybeMatch).trim().split(/layout: /)[1]
+  }
+}
+
+module.exports = {
+  Document,
+}