Tweak .gitignore to always ignore .swiftpm

By default Xcode saves an xctestplan directly in
the `.swiftpm` directory if you change any of its
default values (e.g when enabling code coverage).
As such, ignore the entire `.swiftpm` directly, not
just `.swiftpm/xcode`. Also ignore it for any
child directories, avoiding the need to have separate
`.gitignore`s for child SwiftPM projects.

Author: hamishknight

.gitignore

@@ -2,8 +2,8 @@
 .build/
 # Ignore generated Xcode projects
 *.xcodeproj
-# Ignore Data Xcode stores when opening the project directly
-/.swiftpm/xcode
+# Ignore SwiftPM state, such as the generated xcodeproj when opening the project directly
+.swiftpm
 # Ignore user state in Xcode Projects
 *.xcuserdatad
 UserInterfaceState.xcuserstate