Reorganise the workflow

[giordano]

What analyze(package) now does is the following:

• create a temporary directory (dest)
• clone the repo inside it (dest)
• look for all relevant files in it

I think a more flexible approach would be to

• have an outer function which clones the repo inside the given directory (root/package-name, idea: by default root is a mktempdir, which is automatically deleted at the end of session, but the user can change it)
• have an inner function which only looks for the relevant files inside the given directory (package-name)

For example, PkgEvaluator already has all the packages, so doesn't need to force cloning all them again. Also, this allows keeping all the repo around, to reanalyse them if necessary. As it is now, they're always all thrashed when we're done