Need a better module hierarchy

[yyyliu]

Now the boba and boba-visualizer modules depend on each other ...

The reason that boba depends on boba-visualizer is because we would like a single pip command to install all boba submodules.

The reason that boba-visualizer depends on boba is because the server needs to invoke boba run.

If we separate boba-dsl from boba, and make boba only a dummy module that imports everything else, we can get rid of circular dependency.

Also, I am not sure where to put the sampling code. Maybe we need another module like boba-stats.

[yyyliu]

boba no longer depends on boba-visualizer: uwdata/boba@88aebd9

Users now need to install these two packages separately via two commands, but it might be ok for now ...