Support PHP path entries

This pull request adds the functionality to add ".php" files into the ".pth" files, and will run it. An example usecase is to integrate with other PHP projects and libraries, who often provide an `autoload.php` or `bootstrap.php` file.

From a real-life project:

```sh
$ cat project.pth
src/main/php
src/main/resources
vendor/xp-forge/oauth/src/main/php/
vendor/autoload.php
^^^^^^^^^^^^^^^^^^^
```