Entrypoints should never be ignored

[jbreckmckye]

There is a bug (issue #50) where, if the starting point is a file path(s), it doesn't play nicely with ignore rules like -o / --onlyprefixes. It returns nothing unless you know you have to pass the magic string command-line-arguments:

# Assuming the project entrypoint is ./cmd/main.go
godepgraph -s -o github.com/my/project,command-line-arguments cmd/main.go

This happens because Go's build.Import returns "command-line-arguments" as the ImportPath.

I think the best fix for this is, simply don't ignore packages that are entrypoints. It doesn't make sense to specify a package to traverse and then also ignore it.