Only log once (per phase) when we have to get target distro information from /etc/os-release

[natalieparellano]

Summary

When the run image does NOT have io.buildpacks.base.distro.name or io.buildpacks.base.distro.version labels:

We're currently emitting an info message "target distro name/version labels not found, reading /etc/os-release file" every time we call GetTargetOSFromFileSystem - which happens a LOT, especially during the detect phase. We do this:

• For every buildpack, for every target that the buildpack declares, until we find a target that matches the base image
• For every buildpack, when we run ./bin/detect

This is very noisy, and annoying because it isn't information that buildpack authors need. We should only log the message once, so that platform operators / builder creators can see that the run image needs to be updated with a label.

Release notes

The lifecycle only logs once (per phase) when reading distro information from /etc/os-release in order to construct target information

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Related

Resolves #___

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Context

The updated code is still a bit inefficient, as we should really only be calling GetTargetOSFromFileSystem once per phase, but I've enforced that we only log the info message once, and by saving the distro information the first time we read /etc/os-release, we end up only reading that file once. I didn't want to tear apart too much of the existing code in the interest of time.