Support load after watch

[ktong]

Currently, if Config.Watch is called, any change on a loader loaded after that do not reflect to the configuration because it's not on the watch. This PR removes the chronological order between Config.Load and Config.Watch, so they can be called in different goroutines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.8%. Comparing base (1685aa7) to head (fb363fa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #569   +/-   ##
=====================================
  Coverage   82.8%   82.8%           
=====================================
  Files         62      62           
  Lines       3304    3305    +1     
=====================================
+ Hits        2737    2738    +1     
  Misses       483     483           
  Partials      84      84           

Files with missing lines	Coverage Δ
config.go	96.9% <100.0%> (+<0.1%)	⬆️
internal/nocopy.go	100.0% <100.0%> (ø)
watch.go	100.0% <100.0%> (ø)

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