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
Add support for swift source files
[Doc] Add group for better separation
[Doc] Add some method-inline comments
[Spec][Fixtures] Add a Swift podspec fixture

Never compare that to banana-lib!
Add methods to check if a target uses swift
Support framework as type for generated target
Integrate product_refs of generated frameworks
Migrate from static library to framework
Set headers as public if needed in PodTargetInstaller
Generate an Info.plist for frameworks
Add Swift specific settings to the generated xcconfig files if required
[Spec] Change label of target definition in spec for AggregateTarget

Choose a label for the target definition which contains whitespaces.