Skip generation of label-base BOM

[dmikusa]

Summary

When $BP_BOM_LABEL_DISABLED=true, libpak will throw away label-based BOM information. This PR also looks at that env variable and will skip what is an expensive operation, i.e. fetching all the metadata from the build tool. By default, the label-based BOM is generated so this does not impact that scenario. If specifically disabled, then this will make the build faster.

Use Cases

If you don't want the label-based BOM, then you shouldn't have to wait for it to generate. This PR makes builds faster. It can also work around some bugs. For example, if you map in your Maven directory from your local machine this can cause failures or make things really slow.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).