Don't export whiteouts for single layers

[jonjohnsonjr]

Fixes #1623

Before:

$ crane export quay.io/integreatly/3scale-bundle:v0.8.0 | tar -tv | grep wh
-rwxr-xr-x  0 0      0           0 Dec 31  1969 root/.wh..wh..opq
-rwxr-xr-x  0 0      0           0 Dec 31  1969 metadata/.wh..wh..opq
-rwxr-xr-x  0 0      0           0 Dec 31  1969 manifests/.wh..wh..opq

After:

$ crane export quay.io/integreatly/3scale-bundle:v0.8.0 | tar -tv | grep wh
$

[codecov-commenter]

Codecov Report

Merging #1629 (db05c1a) into main (d958444) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1629      +/-   ##
==========================================
- Coverage   72.58%   72.52%   -0.06%     
==========================================
  Files         119      119              
  Lines        9520     9530      +10     
==========================================
+ Hits         6910     6912       +2     
- Misses       1904     1911       +7     
- Partials      706      707       +1     

Impacted Files	Coverage Δ
pkg/v1/types/types.go	50.00% <0.00%> (-10.00%)	⬇️
pkg/crane/export.go	62.50% <72.72%> (-5.93%)	⬇️

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