Fix base image resolution

[mattmoor]

I noticed that my .ko.yaml was being ignored because it was looking for ko:// prefixed things.

I also tried this with ko publish three ways: ko://-prefixed, non-ko:// prefixed, and relative path, and all picked up the appropriate base.

[codecov-commenter]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   37.82%   37.82%           
=======================================
  Files          33       33           
  Lines        1380     1380           
=======================================
  Hits          522      522           
  Misses        768      768           
  Partials       90       90           

Impacted Files	Coverage Δ
pkg/commands/config.go	15.49% <0.00%> (ø)

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