Remove more files in "make clean" actions.

Examples/Datastore/PackageManager/Makefile

@@ -6,3 +6,4 @@ all:
 clean :
 	rm -rf Packages googleapis .build
 	rm -f Package.pins Datastore Sources/*.pb.swift Sources/swiftgrpc.log google.json
+	rm -rf Package.resolved

Examples/NaturalLanguage/PackageManager/Makefile

@@ -6,3 +6,4 @@ all:
 clean :
 	rm -rf Packages googleapis .build
 	rm -f Package.pins NaturalLanguage Sources/*.pb.swift Sources/swiftgrpc.log google.json
+	rm -rf Package.resolved

Plugin/Makefile

@@ -22,3 +22,4 @@ clear :
 
 clean : clear
 	rm -rf protoc-gen-swiftgrpc Packages .build protoc-gen-swift Package.pins
+	rm -rf Package.resolved