Allow depth control for building ASTs

[object88]

The globals and imports commands traverse the entire AST tree, which may not be what the user is interested in. For example, the user may not care what the stdlib imports.

Suggested traversal levels:

• shallow - just the specified package
• local - just packages that have the same root (i.e., github.com/object88)
• deep - all non-stdlib packages
• complete - every package, regardless of source

Might also be interesting to add a numeric depth control, stopping the traversal after N steps.

[object88]

See imports documentation for provided options.