clean_coverage does not remove *.cov files from the julia directory

[jishnub]

generate_coverage writes out files in both the package src directory as well as the julia installation directory (in /share/julia/base). Running clean_coverage removes the *.cov files from the package, but not from the julia base directory.

[tpapp]

I think it would be nicer not to generate these files at all, so I opened an issue for Pkg.