Allow hostname specific dependency mirrors

[bitgully]

Summary

Enhances the use of dependency mirrors introduced in #315.
Changes have been applied according to the conversation in paketo-buildpacks/rfcs#302 (Readable).

This change allows for setting separate mirror locations for each hostname in the dependencies' original URIs.
A default mirror can still be used for all other dependencies.

Use Cases

This is required in networks where not all dependencies should be pulled from the same mirror location.

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).

[dmikusa]

Thanks for this PR. I added some thoughts/questions. Let me know what you think.

[bitgully]

@dmikusa Would you agree with my responses or do you think this needs more attention?
Any specific ideas how we should go about adding this to packit?

[loewenstein]

Any specific ideas how we should go about adding this to packit?

I think that is already in progress with paketo-buildpacks/packit#563

[dmikusa]

Sorry for the delay. I think this is looking good. Just one last question for you.

Thanks again for your work on this!