Add parallelism when processing multiple files in a dir

### TL;DR

In `copyRecursive()` and in `walkAndModify()`, we use `filepath.WalkDir()` to process each file in a directory sequentially. We could do something like set up a parallel work queue to do IO in multiple goroutines, hopefully improving performance.

We should wait to do this until we have some evidence that our existing approach isn't fast enough.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_