[Installer] Automatically detect conflicts between framework names.

[Init] Output a properly-formed `Podfile` when running `pod init` with a target that contains a `'` in its name.

Closes https://github.com/CocoaPods/CocoaPods/issues/3136.

Closes https://github.com/CocoaPods/CocoaPods/issues/2983.

Build for iOS simulator only during validation

[Outdated] Update podfile sources

Update embed_frameworks_script.rb

Fixed symlink framework issue caused by spaces in `source`

Refs https://github.com/CocoaPods/CocoaPods/issues/3108.

Installation of app-extension targets which had no dependencies
configured in the Podfile would cause an exception before.

Fixes CocoaPods/swift#13

Closes #2996

Configure as extension only for app extensions

Set the APPLICATION_EXTENSION_API_ONLY build setting if integrating with
an app extension target

[IntegrationSpecs] Update for the evaluation of podspecs in the own directory with the podspec option

[PodspecSource] Evaluate :podspec dependencies in their own directory if the path is local.