Change "augmentation library" wording to just "augmentation", other updates

[jakemac53]

A variety of proposed changes related to https://github.com/dart-lang/language/issues/3577.

• Changes references to 'augmentation library' to just 'augmentation', it isn't really a library
• Changes 'import augment' => 'augment' (reads better)
• Changes 'library augment' => 'augment library' (reads better)

I like most all of these changes, possibly with the exception of the augment directive. It feels like it should be augmentation or something, in a perfect world?