Revert "[Analyzer] Allow using CocoaPods with only externally-sourced dependencies in a   `Podfile` without specifying a source or displaying a warning"

This reverts commit 2cc9f01d.

Follow-up to https://github.com/CocoaPods/CocoaPods/commit/7840787e05afb1da24bcfafa4a62f0de41ebf07a.

Conflicts:
	lib/cocoapods/installer/analyzer.rb