Fix SftpAdapter not overwriting on move

[kynx]

Currently the SftpAdapter doesn't overwrite existing files when moving, unlike other adapters. This PR adds that ability.

[kynx]

Moving and copying are both deterministic operations. This means they will always overwrite the target location, and parent directories are always created (if and when needed).

(from https://flysystem.thephpleague.com/docs/usage/filesystem-api/ )

Judging by the failing tests this is based on some alternative facts 😉 But fixing the other adapters should probably be done in separate PRs.

[kynx]

OK, I've moved the moving_a_file_and_overwriting() and moving_a_directory_and_overwriting() tests to SftpAdapterTest so that the other adapter tests no longer fail and this will stand a chance of getting merged. Once the other adapters are fixed these tests should be added back to FilesystemAdapterTestCase

[kynx]

@frankdejonge Apologies, I totally missed that this had conflicts. Those are resolved now.