Exclude helm dependency chart packages from watched files

[Uvedale]

Fixes #931

This excludes files located in the charts subdirectory within a chart package from the list of watched files. This does mean that if someone manually does a helm dep update, the watcher won't pick up the change. However, a dependency version bump would require a change to requirements.yaml, which the watcher will detect.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6ad057a). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #932   +/-   ##
=========================================
  Coverage          ?   43.77%           
=========================================
  Files             ?       63           
  Lines             ?     2641           
  Branches          ?        0           
=========================================
  Hits              ?     1156           
  Misses            ?     1365           
  Partials          ?      120

Impacted Files	Coverage Δ
pkg/skaffold/deploy/helm.go	63.58% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6ad057a...c025a4b. Read the comment docs.

[dgageot]

Thanks @Uvedale! could you add one or a few unit tests? That would be awesome!

[Uvedale]

Ok, I'll see what I can do.

[dgageot]

@Uvedale If you don't have time to write the test, I'm happy to do it.

[Uvedale]

@dgageot I'll add the test(s) tonight. The last few days have been a bit hectic.