performance problems with large filewatch objects

[nicks]

Repro Steps

• Create a Tiltfile in a go project with lots of dependencies
• Run go mod vendor

Current Behavior

tilt creates a very large FileWatch object in-memory

this slows down all operations, e.g.,

time tilt get filewatch -o yaml | tail -n 3
kind: List
metadata:
  resourceVersion: ""

real	0m0.729s
user	0m0.763s
sys	0m0.071s

even though all the data in the filewatch is never used

We should try to consolidate that data.

Additional context
Probably the simplest way to do this would be to add a new field, like "DirectoriesChanged", for consolidating lots of FilesChanged in a single directory.