[Specs] [Pod::UI] First set of unit specs
[UI] Adapt to the removal of the Set::Statistics class
[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`
[Gemfile] Point to Core from master
Merge pull request #2903 from CocoaPods/seg-delete-statistics

[UI] Adapt to the removal of the Set::Statistics class
Validate that a specification's `public_header_files` and `private_header_files` file patterns only match header files.
Also, validate that all file patterns, if given, match at least one file.
Merge pull request #2917 from CocoaPods/seg-validate-file-patterns

Validate that a specification's `public_header_files` and `private_header_files` file patterns only match header files.
Do not generate targets for Pods without sources

If a PodTarget should not be build:
- Do not install a target
- Skip the non-existing native targets in Installer
- Do not add linker flags to XCConfig
Added changelog entry.
Specs for vendored libraries/frameworks.
Improve naming to `should_build?`
Simplify `should_build?`
[Spec] Expect what is documented to happen

Thanks @segiddins.
[Validator] Use header extensions constant from FileAccessor
Merge pull request #2918 from CocoaPods/dont-generate-targets-if-no-source-files

Do not generate targets for Pods without sources