[Specs] [Pod::UI] First set of unit specs
[PodTarget][Migrator] Store private headers in Pod/Headers/Private instead of Build/

Just in case SCM decides to ignore build directories
[CHANGELOG] Add entry for moving private pod headers
Merge pull request #2911 from CocoaPods/seg-store-build-headers-in-private

[PodTarget][Migrator] Store private headers in Pod/Headers/Private instead of Build/
[IntegrationSpecs] Update for move of private headers

Followup to https://github.com/CocoaPods/CocoaPods/pull/2911
[SpecHelper] Consolidate pre-flight setup code
Merge pull request #2845 from CocoaPods/issue/2845

[Unit Tests] Add specs for `Pod::UI`