Use filer in sync command

[pietern]

Changes

This deprecates usage of the repofiles package in favor of the filer package and consolidates the code paths into WSFS.

Note: one breaking change here is the following. If a file at foo/bar.txt is created and removed, the directory foo is kept around because we do not perform directory tracking. If subsequently, we need to write a file at foo, it will result in an fs.ErrExist because it is impossible to overwrite a directory.

The previous implementation performed a recursive delete of the path if this happened, where this implementation will return the fs.ErrExist error to the user.

How to solve directory handling:

1. Maintain set of directories needed for current set of files. Compare to previous set of files. This results in mkdir of added directories and rmdir of removed directories.
2. Making new directories should happen prior to dealing with files. Removing existing directories should happen after dealing with files.
3. Making new directories can be deduped across common prefixes where only the longest prefix is created recursively.
4. Removing existing directories must happen sequentially, starting with the longest prefix.
5. If removal of a directory fails, it must not be empty. We then recursively look for nested empty directories, remove those, and continue. This is necessary to remove empty directories left around by prior sync runs.

Tests

Existing integration tests pass.

[pietern]

We can't merge this without #425. I incorporated the changes in this PR there.