title: baseUrl of tsconfig.json does not work
body: <p>my project structure is
  src/
    client/
    modules/
    server/
      app.ts</p>
  
  <hr>
  
  <pre><code>src/
    client/
    modules/
      DB/
    server/
      app.ts
  tsconfig.json
  </code></pre>
  
  <p>i import a module using non-relative import in app.ts file</p>
  
  <pre><code>import * as DB from '@modules/DB';
  </code></pre>
  
  <p>because i do not want using <code>from "../modules/DB"</code></p>
  
  <p>and my tsconfig.json is</p>
  
  <pre><code>{
  "compileOnSave": true,
  "compilerOptions": {
      "module": "commonjs",
      "target": "ES6",
      "noImplicitAny": true,
      "sourceMap": true,
      "preserveConstEnums": true,
      "emitDecoratorMetadata": true,
      "experimentalDecorators": true,
      "removeComments": true,
      "traceResolution": true,
      "rootDir": "./src",
      "outDir": "./dist",
      "declaration": true,
      "typeRoots": [
          "/"
      ],
      "baseUrl": "./src",
      "paths": {
          "@modules/*": [
              "modules/*"
          ]
      }
  },
  "exclude": [
      "src/client/"
  ]
  </code></pre>
  
  <p>}</p>
  
  <p>when i trace the resolution, got this</p>
  
  <pre><code>======== Module name '@modules/DB' was successfully resolved to '/Users/pengju/projects/pomelo-ts-starter/src/modules/DB/index.ts'. ========
  </code></pre>
  
  <p>it seems to successfuly import</p>
  
  <p>but when compiled, vscode still got wrong with </p>
  
  <pre><code>[ts] Cannot find module '@modules/DB'. 
  </code></pre>
  
  <p>at  <code>import * as DB from '@modules/DB';</code> in app.ts</p>
  
  <p>then i check compiled app.js,
  find <code>const DB = require("@modules/DB");</code></p>
  
  <p>the baseUrl and paths options, cannot change the "@modules/DB" to "../modules/DB"? then,what does they do ?</p>