chore(build): Cleanup firebase build

[jamesdaniels]

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

• Since we use @firebase/* as dependencies now, firebase should only be a dev dependency (we use the development version of the lib it in the tests)
• The typings test should use @firebase/app
• Use the module import from @firebase/app in core

Code sample

n/a

[jamesdaniels]

@davideast I'll cut rc.9 after we merge this.

[jamesdaniels]

Might be related to #1647

[jamesdaniels]

@feryholanda I responded to your issue in #18, please keep the comments related + post general questions to StackOverflow.

[davideast]

LGTM