Removes the dependence on the (global) `ModuleVariableNode.getARead()`,
by adding a local version (that doesn't include `import *` reads)
instead.

This may result in more nodes, but it should still be bounded by the
number of global variables in the source code.

With `ModuleVariableNode`s now appearing for _all_ global variables (not
just the ones that actually seem to be used), some of the tests changed
a bit. Mostly this was in the form of new flow (because of new nodes
that popped into existence). For some inline expectation tests, I opted
to instead exclude these results, as there was no suitable location to
annotate. For the normal tests, I just accepted the output (after having
vetted it carefully, of course).

Overriding both `argumentOf` and `toString` was causing sadness for the
local annotations. Easiest fix was to split these out into their own
subclasses. The names are perhaps a bit too verbose now (alternative
suggestions are welcomed).

We just create one for each call, not just the ones that capture
variables.
In the process, we also made the node synthetic, rather than overriding
the behaviour of (some) `ControlFlowNode`s.