working directory confusion

The way drush messes around with the working directory is really confusing. I found out in #1158490: drush dl --destination should assume a relative path that we can't really rely on the current directory being what it was when the process was started because a bunch of functions mess around with the current working directory. The boostrap process and package managers are the main culprits here.

Now, there's a handy drush_cwd() command to work around that, but you'll notice it doesn't work on windows, because windows doesn't set $PWD when the process start. $PWD is nothing magic - it's inherited by the shell, so we shouldn't expect it to be available.

During the bootstrap of Drupal, we do set the original directory:

../../includes/environment.inc:727:  drush_set_context('DRUSH_OLDCWD', getcwd());

Interestingly enough, drush_cwd() says:

 * TODO: Could cache result, but it isn't really expensive.

This sure looks like a cache to me!

So to summarize here, the problem is as follows:

* getcwd() is not reliable within Drush
* to work around this, we have drush_cwd(), which isn't reliable either (because it relies on PWD)

My suggestion is that we move drush_cwd() to a cache-only operation and configure the cache earlier in the bootstrap. Then we have a more uniform and reliable interface, and we actually make OLDCWD useful.