[PrefixHeader] De-duplicate the contents on the prefix_header file when there are multiple subspecs with the same prefix_header

Closes https://github.com/CocoaPods/CocoaPods/issues/3283.

[CHANGELOG] Used unicode for the emojis (as GitHub emoji codes does not seem to be rendered online) #2892

Fix #3247 by adding quote to $XCASSET_FILES

[Generator] use absolute xcasset file path in copy resource script

Wrong `xcassets` path in generated `Pods-resources.sh` file. This bug
can be reproduced by adding [1PasswordExtension
spec](https://github.com/CocoaPods/Specs/blob/master/Specs/1PasswordExtension/1.1.2/1PasswordExtension.podspec.json)
to demo app.

Include Swift-specific build settings on target creation

Only copy the required xcasset files into the user bundle

[Validator] Allows Swift pods to have a deployment target under iOS 8.0 ...

[XCConfigHelper] Adds $(inherited) to generated OTHER_SWIFT_FLAGS

Before all xcassets used in pod sources where unconditionally integrated into the bundle's car file, even if the xcasset file wasn't referenced by the podspec. Now only files, which are explicitly specified in the spec, will be integrated.

[Resolver] Shows an informative error message when there is no base specification found for a `:head` dependency.

Closes https://github.com/CocoaPods/CocoaPods/issues/3230.

[Validator] Lint dynamic libs for iOS > 7

Fix static binaries check

Allows the usage of `:head` dependencies even when the most recent published version was a pre-release.

Closes https://github.com/CocoaPods/CocoaPods/issues/3212.

Set FRAMEWORK_SEARCH_PATHS only when needed