Get CocoaPods working again

[thomasvl]

Post #1331, pod lib lint SwiftProtobuf.podspec fails because the sources have changed layout.

When we add products to the Package.swift, we'll want subspecs, but for the time being what we conceptually want is probably:

diff --git a/SwiftProtobuf.podspec b/SwiftProtobuf.podspec
index bad55155..ebce3606 100644
--- a/SwiftProtobuf.podspec
+++ b/SwiftProtobuf.podspec
@@ -15,7 +15,7 @@ Pod::Spec.new do |s|
 
   s.cocoapods_version = '>= 1.7.0'
 
-  s.source_files = 'Sources/SwiftProtobuf/**/*.swift'
+  s.source_files = 'Sources/SwiftProtobufCore/*.swift', 'Sources/SwiftProtobufFoundationCompat/*.swift'
 
   s.swift_versions = ['5.0']
 end

However that will still fail because the code expects to be compiled into two modules, which isn't what CocoaPods does even if it were subspecs

fyi @gjcairo @FranzBusch

[thomasvl]

Looking at Firebase, there may be a way to do this by publishing multiple podspecs.