[SourcesManager] Provide a deprecation path for access to Pod::SourcesManager

Fix non-modular header errors for vendored frameworks.

[FileReferencesInstaller] Ensure that headers from vendored frameworks do not end up in the HEADER_SEARCH_PATH when using modules.

[Generator::Plist] Ensure a CFBundleVersion is set for resource bundles

Pass the OTHER_CODE_SIGN_FLAGS variable (aka "Other Code Signing Flags")
to codesign, as Xcode does.

[WIP][PodTargetInstaller] Fix build dir for resource bundles

It was recursively defined before, which led to the target build product being scoped one level to deep.