Prohibit importing from other module 'internal' dirs. (PolymerLabs#5999)

Plus a couple of small fixes to sigh and tsconfig.

tools/custom-loader.mjs

@@ -1,4 +1,5 @@
 import url from 'url';
+import path from 'path';
 
 export function resolve(specifier, parent, resolve) {
   if (!/^[./]/.test(specifier)) {
@@ -12,6 +13,13 @@ export function resolve(specifier, parent, resolve) {
       }
     }
   }
+  if (parent && parent.startsWith('file:///') && specifier.includes('/internal/')) {
+    const parentDir = path.dirname(parent.substr(7));  // 7 == len('file://')
+    const target = path.resolve(parentDir, specifier);
+    if (!target.startsWith(parentDir)) {
+      throw new Error(`cannot access internal file '${specifier}' from external location ${parent}`);
+    }
+  }
   if (!/\.(js|mjs)$/.test(specifier)) {
     if (/build/.test(parent)) {
       const resolved = new url.URL(specifier, parent || 'file:///');

tools/sigh.ts

@@ -391,7 +391,7 @@ function buildPath(path: string, preprocess: () => void): () => boolean {
       return false;
     }
 
-    if (!link(findProjectFiles('src', null, /\.js$/))) {
+    if (!link(findProjectFiles('src', /webpack\.config\.js$/, /\.js$/))) {
       console.error('build::link failed');
       return false;
     }

tsconfig.json

@@ -11,9 +11,7 @@
     "noErrorTruncation": true
   },
   "exclude": [
-    "build/**/*",
-    "cloud/**/*",
-    "./src/tools/language-server/**/*",
-    "./src/tools/vscode-language-client/**/*"
+    "src/tools/language-server/**/*",
+    "src/tools/vscode-language-client/**/*"
   ]
 }