Move call site of `dep_graph_future()`.

Move call site of `dep_graph_future()`.

`Compiler::register_plugins()` calls `passes::register_plugins()`, which
calls `Compiler::dep_graph_future()`. This is the only way in which a
`passes` function calls a `Compiler` function.

This commit moves the `dep_graph_future()` call out of
`passes::register_plugins()` and into `Compiler::register_plugins()`,
which is a more sensible spot for it. This will delay the loading of the
dep graph slightly -- from the middle of plugin registration to the end
of plugin registration -- but plugin registration is fast enough
(especially compared to expansion) that the impact should be neglible.