Stop adding header search paths that do not contain any headers

Revert "Merge pull request #7442 from CocoaPods/seg-vendored-framewor…

This reverts commit d16f648e, reversing
changes made to d7c0b2b3.

Fix legacy header search paths

Use FRAMEWORK_SEARCH_PATHS to discover headers of vendored frameworks

Swift Static Library support

This is to mirror our use of -isystem for header search paths, making it so that headers imported via <> have warnings supressed

By default, targets integrated as static libraries _will not_ get a module map, unless they opt-in by including `DEFINES_MODULE = YES` in their specification's `pod_target_xcconfig`

This allows us to avoid incomplete umbrella header warnings

While well-intentioned, they broke cross-project target dependency detection

xcodebuild fails if a framework target has PRODUCT_MODULE_NAME

Only for static libs at the moment, but this means the relative paths found in the module map will be importable

Because the aggregate target uses the same xcconfig as the user target, we have to disable embedding solely for the target, as it needs to be set to YES in the xcconfig