[CLI] Fix explicit, relative auto-mount for plugins and themes

[brandonpayton]

Motivation for the change, related issues

Providing a relative path to --auto-mount for Plugins and Themes is failing. For example, I am seeing the following when working from a test plugin dir:

npx @wp-playground/cli server --auto-mount=.                                             
Starting a PHP server...
Setting up WordPress latest
Resolved WordPress release URL: https://downloads.w.org/release/wordpress-6.8.3.zip
Fetching SQLite integration plugin...
Booting WordPress...
PHP.request() is deprecated. Please use new PHPRequestHandler() instead.
Booted!
Running the Blueprint...
Activating /wordpress/wp-content/plugins/. – 99%
Plugin /wordpress/wp-content/plugins/. activation printed the following bytes: The plugin does not have a valid header.
Activating /wordpress/wp-content/plugins/. – 100%
Error when executing the blueprint step #0 ({"step":"activatePlugin","pluginPath":"/wordpress/wp-content/plugins/."}) : 
Plugin /wordpress/wp-content/plugins/. could not be activated – WordPress exited with no error. Sometimes, when 
$_SERVER or site options are not configured correctly, WordPress exits early with a 301 redirect. Inspect the "debug" logs in 
the console for more details. caused by Plugin /wordpress/wp-content/plugins/. could not be activated – WordPress exited 
with no error. Sometimes, when $_SERVER or site options are not configured correctly, WordPress exits early with a 301 
redirect. Inspect the "debug" logs in the console for more details.

The main part is:
Error when executing the blueprint step #0 ({"step":"activatePlugin","pluginPath":"/wordpress/wp-content/plugins/."})

This PR updates the --auto-mount code to resolve the path passed to it, so it doesn't matter whether using the empty --auto-mount default or some other relative path. All paths will become absolute paths to avoid these issues.

Implementation details

This PR normalizes all auto-mounted paths to absolute paths before inspection.

Testing Instructions (or ideally a Blueprint)

• CI

[brandonpayton]

I think this change is fine, but I'd like to add some test cases for it first.