Remove redundant identifier in namespace

During my initial work with this component, I wrote this blog post about redundant components in namespaced symbols.

I see several places in this component where we can improve the use of namespaces.

I do want to specifically mention one case, however. The exceptionality of this case wasn't obvious to me at first: Drupal\DrupalExtension.
You can see in the blog post that my initial thought was that Drupal\Extension would be more appropriate. However, I don't want to "go there" presently, because I recognize the pattern of Drupal\, and that's fine.

However, there are still several cases where this principle can be employed. Consider the following fully-qualified symbols in this project:

\Drupal\DrupalExtension\Definition\Proposal\AnnotatedDefinitionProposal
\Drupal\DrupalExtension\Context\ClassGuesser\DrupalContextClassGuesser
\Drupal\DrupalExtension\Selector\RegionSelector
\Drupal\DrupalExtension\Context\DrupalContext
\Drupal\DrupalExtension\Context\Initializer\DrupalAwareInitializer

For what it's worth, I want to mention what I see as exemplary of what I am suggesting:

\Drupal\DrupalExtension\Compiler\DriverPass

There are two direction we can go with this:

1. fewer namespaces, verbose local symbols
2. lightweight local symbols via hierarchy of namespaces

An example of the first technique:

\Drupal\DrupalExtension\Definition\Proposal\AnnotatedDefinitionProposal
// becomes
\Drupal\DrupalExtension\AnnotatedDefinitionProposal

An example of the second:

\Drupal\DrupalExtension\Definition\Proposal\AnnotatedDefinitionProposal
// becomes
\Drupal\DrupalExtension\Definition\Proposal\Annotated

Both of these remove unnecessary redundancy in the names.
The choice can be made based on a number of factors, but the second is *probably* better if we have chosen decent component names already. In fact, if that's a given, I'll just say that the second way is my clear preference. I'll save the discussion between \Definition\Proposal\ and \DefinitionProposal\ for another day.