[9.x] Fix directory for nested markdown files for mailables

[ahinkle]

This PR allows for the correct location on nested markdown mailables. Useful for applications with many mailables.

Currently, if you create a mailable with a nested location, it will generate a mailable class in the nested directory. However, the markdown will be in the mail directory and not within the nested directory.

php artisan make:mail Registration/WelcomeEmail -m

// generates:
app\Mail\Registration\WelcomeEmail.php
resources\views\mail\welcome-email.blade.php

This PR will place the blade in the proper nested directory.

Proposed:

php artisan make:mail Registration/WelcomeEmail -m

// generates:
app\Mail\Registration\WelcomeEmail.php
resources\views\mail\registration\welcome-email.blade.php

of course, you can manually override the location with --markdown=(location) but this fixes the directory when an empty markdown location is provided.

I didn't see any tests for this but I tested this thoroughly locally. Thanks!