feat(yarn): support yarn offline mirror (#4102)

Look for .yarnrc in same directory as yarn.lock. If found, look for yarn-offine-mirror line and check that folder for changes.

Closes #1452

lib/manager/npm/post-update/index.js

@@ -482,6 +482,45 @@ async function getAdditionalFiles(config, packageFiles) {
           name: lockFileName,
           contents: res.lockFile,
         });
+        // istanbul ignore next
+        try {
+          const yarnrc = await platform.getFile(
+            upath.join(lockFileDir, '.yarnrc')
+          );
+          if (yarnrc) {
+            const mirrorLine = yarnrc
+              .split('\n')
+              .find(line => line.startsWith('yarn-offline-mirror '));
+            if (mirrorLine) {
+              const mirrorPath = mirrorLine
+                .split(' ')[1]
+                .replace(/"/g, '')
+                .replace(/\/?$/, '/');
+              const resolvedPath = upath.join(lockFileDir, mirrorPath);
+              logger.info('Found yarn offline  mirror: ' + resolvedPath);
+              const status = await platform.getRepoStatus();
+              for (const f of status.modified.concat(status.not_added)) {
+                if (f.startsWith(resolvedPath)) {
+                  const localModified = upath.join(config.localDir, f);
+                  updatedArtifacts.push({
+                    name: f,
+                    contents: await fs.readFile(localModified, 'utf8'),
+                  });
+                }
+              }
+              for (const f of status.deleted || []) {
+                if (f.startsWith(resolvedPath)) {
+                  updatedArtifacts.push({
+                    name: '|delete|',
+                    contents: f,
+                  });
+                }
+              }
+            }
+          }
+        } catch (err) {
+          logger.error({ err }, 'Error updating yarn offline packages');
+        }
       } else {
         logger.debug("yarn.lock hasn't changed");
       }