resolving TUF target name for project name

[jku]

Current release pip downloads https://pypi.org/simple/sampleproject when it wants the sampleproject index file. This won't work for TUF since we need a specific version of the index file.

This is being fixed in warehouse pypi/warehouse#8487 but the exact URL (and thus exact target name in TUF metadata) is not decided yet.

My work-in-progress banch uses {PROJECT}/index.html as target name which TUF then expands to https://pypi.org/simple/{PROJECT}/{HASH}.index.html for the download.