cmd/image/tree: refactor

[laurazard]

- What I did

Just some refactors to the image tree formatting logic to make reuse/reading easier. Consolidates the printing in one place, and reduces how far down we need to pass our writer.

This also makes testing these functions less involved.

- How I did it

(wip, I need to push more commits)

Make most of the formatting functions return strings, instead of printing directly.

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 195 lines in your changes missing coverage. Please review.

Project coverage is 59.55%. Comparing base (da9e984) to head (6c86dcc).
Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5572      +/-   ##
==========================================
+ Coverage   59.15%   59.55%   +0.39%     
==========================================
  Files         342      346       +4     
  Lines       29079    29119      +40     
==========================================
+ Hits        17203    17341     +138     
+ Misses      10901    10808      -93     
+ Partials      975      970       -5