Improve kustomization traversal performance

[allenporter]

Improve traversal performance. Speed up by running kustomizations in parallel again, which is now possible after previous simplifications. Even for a small cluster we see a large impact win:

Old:

# time flux-local get hr --path tests/testdata/cluster
NAME            REVISION    CHART                       SOURCE          
weave-gitops    4.0.22      flux-system-weave-gitops    weave-charts    

real    0m4.229s
user    0m5.476s
sys     0m2.140s

New:

# time flux-local get hr --path tests/testdata/cluster
NAME            REVISION    CHART                       SOURCE          
weave-gitops    4.0.22      flux-system-weave-gitops    weave-charts    

real    0m0.996s
user    0m2.304s
sys     0m0.376s

For my own cluster before

$ time flux-local get hr -A --path clusters/prod/flux-system/
...
real    0m14.109s
user    0m19.698s
sys     0m8.298s

After:

$ time flux-local get hr -A --path clusters/prod/flux-system/
...
real    0m5.313s
user    0m10.584s
sys     0m2.383s

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (c50ba40) 92.68% compared to head (f1fe16a) 92.79%.

Files	Patch %	Lines
flux_local/git_repo.py	87.03%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
+ Coverage   92.68%   92.79%   +0.10%     
==========================================
  Files          18       18              
  Lines        1681     1706      +25     
==========================================
+ Hits         1558     1583      +25     
  Misses        123      123              

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