4073 [image_picker] Migrate image_picker to package:cross_file (cla: yes, waiting for tree to go green, p: image_picker)

4083 [image_picker_for_web] Migrate image_picker to package:cross_file (cla: yes, p: image_picker, platform-web)

4089 [image_picker] Updated pickImage and pickVideo docs to expose the possible errors that can be thrown (cla: yes, waiting for tree to go green, p: image_picker)

4103 [image_picker]Update example app (cla: yes, waiting for tree to go green, p: image_picker)

4117 [image_picker] Fixed IOException when cache directory is removed (cla: yes, waiting for tree to go green, p: image_picker, platform-android)