Fix the feature module template writing the impl build script into the api module

Generating a feature with the Generate Module from Template plugin produced a
module that could not build: features/<name>/api/build.gradle.kts was written
from the impl template, giving the api module a dependency on itself and an
impl namespace.

FeatureModule.json declared BUILD_GRADLE_API and BUILD_GRADLE_IMPL, both
defaulting to build.gradle.kts, and used them as fileTemplates keys. The plugin
resolves keys to filenames before matching, so both collapsed to one key and
the second overwrote the first. The same matching code falls back to a suffix
match on the path, so DefaultXEntryPoint also matched the key XEntryPoint and
was generated from the api template.

Key both by directory, which the plugin supports for exactly this case, and
drop the two now-unused placeholders. The file templates themselves were
correct and are unchanged. The onboarding doc gains a note that an already
imported template must be removed before re-importing, since the plugin keeps
its own copy.

docs/_developer_onboarding.md

@@ -303,6 +303,8 @@ Follow these steps to install and configure the plugin and templates:
    - Click on OK
 4. Configure generate-module-from-template plugin :
    - Navigate to AS/Settings/Tools/Module Template Settings
+   - If a `FeatureModule` template is already listed, select it and remove it, since the plugin keeps its own copy of the
+     imported template and will not pick up changes made to `FeatureModule.json` on its own
    - Click on + / Import From File
    - Pick the `tools/templates/FeatureModule.json`
 
@@ -321,8 +323,10 @@ Example for a new feature called RoomDetails:
 5. The modules api/impl should be created under `features/roomdetails` directory.
 6. Sync project with Gradle so the modules are recognized (no need to add them to settings.gradle).
 7. You can now add more Presentation classes (Events, State, StateProvider, View, Presenter) in the impl module with the `Template Presentation Classes`.
-   To use it, just right click on the package where you want to generate classes, and click on `Template Presentation Classes`.
-   Fill the text field with the base name of the classes, ie `RootRoomDetails` in the `root` package.
+   To use it, just right click on the package where you want to generate classes, and click on `New` / `Template Presentation Classes`.
+   Fill the text field with the base name of the classes: the generated files are `<BaseName>Event.kt`, `<BaseName>State.kt`,
+   `<BaseName>StateProvider.kt`, `<BaseName>Presenter.kt`, `<BaseName>Node.kt` and `<BaseName>View.kt`. For instance
+   `PreferencesRoot` in the `features/preferences/impl/root` package.
 
 
 Note that naming of files and classes is important, since those names are used to set up code coverage rules. For instance, presenters MUST have a