fix(prepare): mapImageResources always returning []

[raphinesse]

Motivation and Context

I broke mapImageResources when refactoring it to use globbing in #1085. The problem is, that fast-glob will only match files by default while we are looking for directories here.

Description

Pass onlyDirectories option to fast-glob.

[codecov-io]

Codecov Report

Merging #1136 (b4aec3d) into master (97e2d15) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1136      +/-   ##
==========================================
- Coverage   71.19%   71.14%   -0.05%     
==========================================
  Files          21       21              
  Lines        1739     1740       +1     
==========================================
  Hits         1238     1238              
- Misses        501      502       +1     

Impacted Files	Coverage Δ
bin/templates/cordova/lib/prepare.js	43.76% <0.00%> (-0.15%)	⬇️

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