Enable extension developers to customize the namespace
Enable extension developers to customize the namespace

If you are developing an extension and you want to use
`Rake::Task['extension:test_app']` you are forced to have a file named
exactly as the capitalized version of the extension namespace name, so
if you name your namespace extension e.g. SolidusGraphQL you are forced
to create a file named `solidus_graph_q_l`, otherwise the following
error is raised:

```
> bundle exec rake test_app --trace
** Invoke default (first_time)
** Invoke first_run (first_time)
** Execute first_run
** Invoke test_app (first_time)
** Execute test_app
** Invoke extension:test_app (first_time)
** Execute extension:test_app
** Invoke common:test_app (first_time)
** Execute common:test_app
rake aborted!
LoadError: cannot load such file -- solidus_graph_q_l
```

 This change allows the developer to customize the extension namespace
setting a `LIB_NAMESPACE` environment variable.

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus