Dont calculate buildpack layer diffIDs twice when creating a builder

[ekcasey]

Signed-off-by: Emily Casey ecasey@vmware.com

Summary

Provides the buildpack layer diffIDs to imgutil so imgutil can avoid recalculating them.

Builder creation without --publish is annoyingly slow for a large builder. This only saves a couple seconds for the largest paketo builder. Other performance improvements would be more impactful such as:

• Not calculating the layer diffID at all when we know it (e.g. when we are pulling a buildpack from a buildpackage)
• Local Layer Cache #866

But I thought I would contribute this after noticing it because it is a trivial change the preserve the existing behavior.

Output

Before

Using https://github.com/paketo-buildpacks/full-builder:

> time pack builder create bldr/full -c ./builder.toml --pull-policy=never
pack builder create bldr/full -c ./builder.toml --pull-policy=never  11.32s user 5.38s system 45% cpu 36.312 total

After

> time pack builder create bldr/full -c ./builder.toml --pull-policy=never
pack builder create bldr/full -c ./builder.toml --pull-policy=never 9.71s user 4.84s system 40% cpu 36.326 total

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

[codecov]

Codecov Report

Merging #1080 (7ac6cdb) into main (0bd1584) will not change coverage.
The diff coverage is 20.00%.

@@           Coverage Diff           @@
##             main    #1080   +/-   ##
=======================================
  Coverage   80.35%   80.35%           
=======================================
  Files         130      130           
  Lines        7964     7964           
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  Hits         6399     6399           
  Misses       1147     1147           
  Partials      418      418           

Flag	Coverage Δ
os_linux	79.67% <20.00%> (ø)
os_macos	77.11% <20.00%> (-0.03%)	⬇️
os_windows	80.27% <20.00%> (ø)
unit	79.72% <20.00%> (ø)

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