Implement package weight for deepdep

[ErdaradunGaztea]

Package weight could be a parameter of deepdep() function or a function applied to a deepdep object. I don't like parameter bloat, so I'd opt for the latter solution. This would work like that:

dd <- deepdep("ggplot2", depth = 2)
# Function name is definitely open for a discussion
dependency_weights(dd)

This would allow us to implement something like "package dependency score" as well, that would grade packages... But that's a separate idea.

Weights themselves may be computed in more than one way; it could be size of a package or a number of exported functions (probably with S3 methods not being counted as separate).