Update .gitignore for Swift Package Manager (flutter#6705)

The following directories were created when I added SPM support to the `ios_platform_images` plugin:

```
packages/ios_platform_images/ios/ios_platform_images/.build/
packages/ios_platform_images/ios/ios_platform_images/.swiftpm/
```

These shouldn't be checked-in to the packages repository.

Part of flutter/flutter#148018

Author: loic-sharma

.gitignore

@@ -12,8 +12,11 @@
 
 pubspec.lock
 
+# iOS and macOS dependencies
+.build/
 Podfile.lock
 Pods/
+.swiftpm/
 .symlinks/
 
 *instrumentscli*.trace