fix: hardcoded '/' in normalized options.fs, fixes #412, #408

Changes including:
- will not call fs.resolve when normalizing `fs`
- removed hardcoded `/`
- mandatory `fs.dirname` for relative reference
- mandatory `fs.sep`, defaults to '/'

test/unit/fs/loader.ts

@@ -12,5 +12,15 @@ describe('fs/loader', function () {
       const candidates = [...loader.candidates('./foo/bar', ['/root', '/root/foo'], '/root/current', true)]
       expect(candidates).to.contain('/root/foo/bar')
     })
+    it('should not include out of root candidates', async function () {
+      const loader = new Loader({ relativeReference: true, fs, extname: '' } as any)
+      const candidates = [...loader.candidates('../foo/bar', ['/root'], '/root/current', true)]
+      expect(candidates).to.have.lengthOf(0)
+    })
+    it('should treat root as a terminated path', async function () {
+      const loader = new Loader({ relativeReference: true, fs, extname: '' } as any)
+      const candidates = [...loader.candidates('../root-dir/bar', ['/root'], '/root/current', true)]
+      expect(candidates).to.have.lengthOf(0)
+    })
   })
 })