Currently watching files uses polling which is inefficient with large codebases. On my laptop ./mill --watch uses 20% CPU just to watch the files for changes.

This uses oslib's watch module to watch for the file changes instead of polling them.

We had to update oslib (com-lihaoyi/os-lib#386) to:

• add a capability of filtering what folders it watches. This is needed because watching large generated folders like out or .bloop emits Java FS watcher OVERFLOW events.
• remove the random println's that oslib.watch had.

The current approach to watching is to watch the workspace root via FS watching. FS watching only works with folders and we have to watch root/build.mill, and thus, root/, so everything else falls under it.

Mac OS watcher performs watching recursively natively, but on linux oslib adds recursive watches itself, so we use oslib.watch filter to prevent watching anything that is not an ancestor of watched root. For example, if we have source at root/module-a/src/, we'll watch root/, root/module-a/ and root/.

Finally, events are filtered so that unrelated changes (like creating root/random-file.txt) does not trigger reevaluation.

The fs watching is enabled by default and can be disabled via --notify-watch=false.

0.12.x version: #5073