Update Blueprint repository to match method definitions with core

[ryanmitchell]

Fixes: #92, Fixes #93

Reference PR in core: https://github.com/statamic/cms/pull/6588

[what-the-diff[bot]]

• Changed the visibility of three constants from private to protected.
• Removed two blank lines in between methods and one at the end of a method body.
• Replaced four instances of 'blueprints' with $this->getHandle(). This is because this class extends StacheRepository, which has getHandle() defined as returning 'blueprints'.