Added support for relative paths inside proxies in generated index.yaml

[tmckeag]

This pull request makes the following changes:

• Makes the relative-URL writing in the proxy index generator (IndexAbsoluteUrlRewriter) aware of the URL of the remote repository.
• Changes URL rewrite to add appropriate consideration for relative paths inside the proxied repository.
• Adds a test case in the relative .yaml test resource to ensure behavior is correct.

It relates to the following issue #s:

• Helm proxy doesn't work for repos with internal relative paths #127

[sonatypecla]

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[sonatypecla]

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[mlukaretkyi]

@tmckeag
Hi, thanks for your PR. Could you take a look at this issue?
#94
As far as I understand you try to fix this issue, but I think we need to implement this in a different way.
I think you need to take a look at HelmProxyFacetImpl.getCachedContent. My guess, you can rewrite URLs in this method