Move path.inc functions into OOP autoloaded potential code to avoid WSOD when calling HandlerPathSystem::getValue() [statistics]

This bug is related to the #1233232: Add unified context system to core issue.

The only core test case that doesn't pass following the WSCCI/Context API core inclusion is the Statistics module failing to pass test when a cached page is served.

Here is the real cause of the bug:

1. Statistic module's hook_exit() calls arg()
2. arg() calls drupal_get_context()->get('path:system')
3. This calls HandlerPathSystem::getValue()
4. ... which calls drupal_get_normal_path()
5. drupal_get_normal_path() is defined in the path.inc that isn't included yet.
6. WSOD happens

The worst part is this error remains silent, because _drupal_bootstrap_page_cache() already sent the headers and the full cached content, browsers received everything and this is run after. The crash is not a WSOD, because HTML is fully sent, but the error is fatal so PHP don't send the notice.

The only way to see it is in your HTTPd/PHP error log.

Now there is three methods to fix it, in order from the ugliest to the nicest:

1. Add a pragmatic require_once inside the statistic module's hook_exit() implementation: this is ugly and it doesn't solve the potential bug for all other modules.
2. A compromise temporary quick solution is to do the same require_once inside the HandlerPathSystem::getValue() method. Yet again still not really nice.
3. Best solution is to refactor the path handling using OO code and let the autoloader do its magic: this need more than a one line path and need its own issue if that is the way to go.

What do you think?