[BUGFIX] PHP Notice when multiple namespace roots are set. (Revision 1)

The Package class triggers a PHP Notice when a composer manifest
contains several search paths for the same prefix (see [1]):

    {
      "autoload": {
        "psr-0": {
          "Foo\\": ["src/", "tests/"]
        }
      }
    }

This commit changes the behaviour to using the first path as class
path when multiple paths are defined.

[1] https://getcomposer.org/doc/04-schema.md#psr-0

Change-Id: If33ba0cb4b3dd22c8543e2ee5f881967f342c372
Releases: master
Revision-Id: a721f9eca3a7665a03fe65c74be4518a31922f94