refactor(core): Implement our own polling file watcher

[timoffex]

Description

Replaces the radovskyb/watcher by our own implementation with important advantages.

In the following snippet, onChange1 is guaranteed to be invoked and onChange2 is guaranteed to not be invoked:

watcher.Watch(path, onChange1)
modifyFile(path) // assume this changes mtime or file size
watcher.Watch(path, onChange2)

Another difference is that Watch and WatchTree work even if the path does not point to an existing file or directory.

The parallelism is also different: instead of a single goroutine looping over all watched files, there is one goroutine per watched path. This is primarily for simplicity, but it also prevents a watcher for a large file tree from slowing down other watchers. Though there is only one goroutine per path, that goroutine may own multiple callbacks which minimizes the number of Stat() calls per path.

[codecov]

Codecov Report

Attention: Patch coverage is 90.56604% with 15 lines in your changes missing coverage. Please review.

Project coverage is 73.01%. Comparing base (6bc85c2) to head (635cad9).
Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7674      +/-   ##
==========================================
+ Coverage   72.81%   73.01%   +0.19%     
==========================================
  Files         491      502      +11     
  Lines       55565    54240    -1325     
==========================================
- Hits        40462    39604     -858     
+ Misses      14681    14226     -455     
+ Partials      422      410      -12     

Flag	Coverage Δ
func	41.36% <ø> (+0.04%)	⬆️
system	63.53% <70.19%> (+0.03%)	⬆️
unit	52.08% <89.30%> (+1.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/internal/waiting/waiting.go	81.39% <100.00%> (+9.30%)	⬆️
core/internal/waitingtest/fakestopwatch.go	79.41% <100.00%> (ø)
core/internal/watcher/filemodtoken.go	100.00% <100.00%> (ø)
core/internal/watcher/watcher.go	100.00% <ø> (ø)
core/pkg/server/tb.go	22.41% <0.00%> (+2.69%)	⬆️
core/internal/watcher/watcherimpl.go	95.83% <95.83%> (ø)
core/internal/watchertest/watchertest.go	50.00% <33.33%> (ø)
core/internal/watcher/treewatcher.go	91.48% <91.48%> (ø)

... and 145 files with indirect coverage changes