♻️ Improve template structure

We had set up the template to have a `{{ plugin_name }}` directory at the root of the
`template` directory. The idea was that a user can just `copier copy` the template in
the current directory and it would automatically create a full directory structure for
the plugin package, where the root directory of the package would always match the
module directory in the `src`.

However, this creates issues when we want to start using `copier update`. Ideally, we'd
be able to use the previous answers stored in the `.copier-answers.yml` file. However,
this is created in the root directory of the template by `copier`. So it would be
created in the parent directory of our actual package with the current set up. This
means a user can not just go inside the `plugin_name` directory and run `copier update`.

We've looked for workarounds based on copying the `.copier-answers.yml` file inside the
`plugin_name` directory post `copier copy`, but they all were deemed messy, fragile and
perhaps even not fully functional. Instead, we restructured the template by:

1. Removing the `{{ plugin_name }}` subdirectory in the `template` directory, instead
   moving its contents into the root `template` directory.
2. Using the `copier copy` `destination_path` (retrievable via `_copier_conf.dst_path`)
   for the default of the `plugin_name` input.

This means the user can now run:

  copier copy https://github.com/mbercx/aiida-plugin-copier <plugin_name>

and use the defaults to generate a new plugin package in the `<plugin_name>` directory.

Author: mbercx

README.md

@@ -8,7 +8,7 @@ Follow the [`copier` installation instructions](https://copier.readthedocs.io/en
 Then simply run
 
 ```
-copier copy https://github.com/mbercx/aiida-plugin-copier .
+copier copy https://github.com/mbercx/aiida-plugin-copier <plugin_name>
 ```
 
 And answer the questions to generate a new AiiDA plugin package in the current directory.

copier.yml

@@ -1,6 +1,7 @@
 plugin_name:
   type: str
   help: 'What is the name of your plugin package?'
+  default: '{{ _copier_conf.dst_path | basename }}'
 
 description:
   type: str

docs/index.md

@@ -8,7 +8,7 @@ Follow the [`copier` installation instructions](https://copier.readthedocs.io/en
 Then simply run
 
 ```
-copier copy https://github.com/mbercx/aiida-plugin-copier .
+copier copy https://github.com/mbercx/aiida-plugin-copier <plugin_name>
 ```
 
 And answer the questions to generate a new AiiDA plugin package in the current directory.

pyproject.toml

@@ -4,18 +4,18 @@ dependencies = [
   "uv"
 ]
 scripts.copy = [
-  "copier copy . {args:.tmp} -f -d plugin_name=aiida-test",
+  "copier copy . {args:.tmp} -f",
 ]
 scripts.install = [
-  "copier copy . .tmp -f -d plugin_name=aiida-test",
+  "copier copy . .tmp/aiida-test -f",
   "uv pip install .tmp/aiida-test",
 ]
 scripts.check = [
-  "copier copy . .tmp -f -d plugin_name=aiida-test",
+  "copier copy . .tmp/aiida-test -f",
   "hatch fmt -l --check .tmp/aiida-test",
 ]
 scripts.docs = [
-  "copier copy . .tmp -f -d plugin_name=aiida-test",
+  "copier copy . .tmp/aiida-test -f",
   "cd .tmp/aiida-test/docs && myst start",
 ]
 scripts.clean = [