fix(Turborepo): Make package discovery async, and apply a debouncer (#8058)
### Description
- move the debouncer to its own module so that it is reusable
- apply the debouncer to package discovery
- make package discovery async so it doesn't block the file watching
channel
### Testing Instructions
Existing test suite.
Fixes #3455
Closes TURBO-2909
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Co-authored-by: Greg Soltis <Greg Soltis>
Co-authored-by: Alexander Lyon <arlyon@me.com>