Expose __transitive_delta__ in environment.

Each target's local environment, after deps processing, now contains
the transitive closure of its dependency environment.  I intend this
to be used to make assertions or inspections of the DAG.
Allow modules without package_verbs.
Allow modules with global_functions.

These functions get exposed in the loader environment without any
argument binding -- unlike package_verbs, which get bound to useful
hidden state.
Add concept of target checks.

These are lists of functions (or target-specific methods) run when
a target is evaluated in an environment.  They have access to the
environment after 'using' is applied, which allows them to check
properties of the environment -- for example, to verify that no
transitive dependence on a certain target has been introduced.