[Snapshot] Move files to src (#6138)

### Context
In working on #6137 I noticed snapshot test helpers were in the private directory but weren't in a _src_ folder. This doesn't align with any of our other components in the _private_ folder.
### The problem
Snapshot test helpers weren't in a _src_ folder.
### The fix
Move the files into _src_.
### Additional notes
I have removed the BUILD file as we will need to add a BUILD file to [ios-snapshot-test-case](https://github.com/uber/ios-snapshot-test-case) before we can add a BUILD file.

MaterialComponentsSnapshotTests.podspec

@@ -55,7 +55,7 @@ Pod::Spec.new do |s|
   s.subspec "private" do |private_spec|
     private_spec.test_spec "Snapshot" do |snapshot|
       snapshot.ios.deployment_target = '8.0'
-      snapshot.source_files = "components/private/#{snapshot.base_name}/*.{h,m,swift}"
+      snapshot.source_files = "components/private/#{snapshot.base_name}/src/*.{h,m,swift}"
       snapshot.dependency 'iOSSnapshotTestCase', '2.2.0'
     end
   end