`repack-stemcell` stops prefixing archive members with "./"

[selzoc]

Prior to this commit, running bosh repack-stemcell would result in a repacked stemcell with archive members:

./
./dev_tools_file_list.txt
./image
./stemcell.MF
./packages.txt

while the original stemcell has archive members:

dev_tools_file_list.txt
image
packages.txt
stemcell.MF

This commit preserves the original stemcell archive members in the repacked stemcell. This is useful if you're trying to extract a particular file from the archive by name.

In addition, this commit:

• Vastly simplifies the repack-stemcell command tests (including removing an entire compressor fake separate from the existing one in bosh-utils)
• Removes unneeded UI and FS depedencies from the repack-stemcell command

[selzoc]

@jpalermo you're listed as a reviewer here, can you take a look and/or merge?