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.

[SamMousa]

@kynx I'm gonna try and get posix_rename support in PHPSecLib.

phpseclib/phpseclib#2024

Just an FYI. If that works out we should support it in this adapter as well (and fallback to the manual solution if not supported).

Edit:
I've also implemented a generic solution for all adapters here: collecthor/flysystem-adapters#7
Of course this is suboptimal but it provides a path forward at least.

[kynx]

@SamMousa great idea with the posix_rename stuff! I've always felt a little nervous around the phpseclib code 😄

The generic adapter solution is also a worthwhile compromise - I should have thought of something like that.

TBH I'd completely forgotten this PR, but just checked and oops - it's been in production on one project for over a year! Got to find some tool to send reminders when something lurks in composer's' repositories too long 😉

[SamMousa]

@kynx posix_rename has been merged: phpseclib/phpseclib#2029

Are you up for adding support to the sftp adapter?