## Changes
This PR:
1. Adds the export-dir command
2. Changes filer.Read to return an error if a user tries to read a
directory
3. Adds returning internal file structures from filer.Stat().Sys()
## Tests
Integration tests and manually
## Changes
This captures the recursive deletion of a directory tree in the filer interface.
Prompted by #433.
## Tests
Integration tests pass (ran the filer ones on AWS and Azure).
Adds a DBFS implementation of the `filer.Filer` interface.
The integration tests are reused between the workspace filesystem and
DBFS implementations to ensure identical behavior.
Summary:
* All remote path arguments for deployer and locker are now relative to
root specified at initialization
* The workspace client is now a struct field so it doesn't have to be
passed around
shreyas-goenka
Pieter Noordhuis