This uses move_atomic insteand of the manual os.rename and manual error
checking and throws a SourceError for consistency with other modules.

Moving atomically a file/directory can be tricky since different
errors might be raised for the same underlying problem.

Having a utility function to reduce this discrepancies will help
in ensuring we have correct behavior

According to the documentation
(https://www.unix.com/man-page/POSIX/3posix/rename/), when the directory
already is there, either EEXIST or ENOTEMPTY could be thrown.

Previously only ENOTEMPTY was checked.

Done:
  - Separated the move into its own function
  - Check for both errors
  - Create unit tests for it, covering most test cases

Previously the result of the first call in test_build_track was not
checked to be successful, which would lead to potential erros later on
that were hard to debug.

Let's check it everytime

source.py: don't let StopIteration propagate to silence() contextmanager

See merge request BuildStream/buildstream!945

As per PEP 0479 (https://www.python.org/dev/peps/pep-0479/),
StopIteration thrown in context managers are not valid starting from
Python 3.7.

Direct CAS-to-CAS import

Closes #574

See merge request BuildStream/buildstream!911

This will now return paths in the same order as list_relative_paths.