👌 Simplify plugin_name default value

We were using `{{ _copier_conf.dst_path | basename }}` for the default of `plugin_name`
variable. However, this has several issues:

1. When the user tries to copy the template in the current directory (`.`), the `.` also
   shows up as the default.
1. The `dst_path` may be a temporary directory when updating projects. This might be
   an issue down the line.

Turns out `copier` defines a global variable `_folder_name` that gives the name of the
(generated) project root directory. This means the default can be simplified, and we
don't face the issues described above.

Author: mbercx

copier.yml

@@ -1,8 +1,7 @@
 plugin_name:
   type: str
   help: 'What is the name of your plugin package?'
-  default: '{{ _copier_conf.dst_path | basename }}'
-
+  default: '{{_folder_name }}'
 description:
   type: str
   help: 'Describe what your plugin package is for.'