plug-ins: fix C goat-exercise build/installation in meson.

It was installing in separate dirs `goat-exercise/goat-exercise` and
`goat-exercise-c/goat-exercise-c.c`. This is obviously wrong as the C
raw file is obviously not a plug-in of its own, it is just a data for
the built plug-in to be displayed as source.
Anyway now fixed to work like the autotools build.

plug-ins/goat-exercises/meson.build

@@ -1,4 +1,6 @@
-plugin_name = 'goat-exercise'
+# C version
+
+plugin_name = 'goat-exercise-c'
 
 plugin_sources = [
   'goat-exercise-c.c',
@@ -37,12 +39,13 @@ executable(plugin_name,
   install_dir: gimpplugindir /  'plug-ins' / plugin_name,
 )
 
-
 install_data(
   'goat-exercise-c.c',
   install_dir: gimpplugindir /  'plug-ins' / plugin_name + '-c',
 )
 
+# Python 3 (pygobject) version.
+
 if have_python
   install_data(
     'goat-exercise-py3.py',
@@ -50,6 +53,7 @@ if have_python
   )
 endif
 
+# Javascript (GJS) version.
 
 if have_javascript
   install_data(
@@ -58,6 +62,8 @@ if have_javascript
   )
 endif
 
+# Lua (lua-jit + LGI) version.
+
 if have_lua
   install_data(
     'goat-exercise-lua.lua',