Allow symlinked plugins

[scribu]

There are many scenarios where one would like to have a plugin's folder symlinked to another location.

A couple of these scenarios are described in #13550.

However, when using symlinks, code such as this fails:

plugins_url( 'script.js', __FILE__ );

This happens because __FILE__ resolves to the real path, which confuses plugin_basename().

The most simple and most flexible solution is to add a filter to plugin_basename(), leaving individual devs to handle symlinked paths, depending on their environment.

'pre_plugin_basename' filter

Test files coming up.

To test:

1. Extract test.zip​ into a wp root directory
2. Activate Test Plugin

The archive already contains a symlink, so it probably won't work on Windows out of the box.