Simplify command

for the moment I didn't have better idea how to shrink the contents of dependencies.file but to filter lines where a package name belongs to the set of indirect packages.
this probably needs more test cases and docs and stuff
yeah and move https://github.com/fsprojects/Paket/blob/master/src/Paket/InstallProcess.fs#L71-L76 to ProjectFile as a class member to reuse it