## Changes
This enables the use of `io/fs` functions `fs.Glob` and `fs.WalkDir`
with filers.
We can't use `fs.FS` as the standard interface instead of `filer.Filer` because:
1. It was made for reading from filesystems only, not writing
2. It doesn't take a context for the core functions
Therefore a wrapper will do.
## Tests
* Added unit tests to cover the adapter through a fake filer.
* Manually ran `fs.WalkDir` against both WSFS and DBFS filers.
## Changes
The pattern `errors.Is(err, fs.ErrNotExist)` is common to check for an
error type.
Errors can implement `Is(error) bool` with a custom equivalence checker.
## Tests
New asserts all pass in the integration test.
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
shreyas-goenka