rustc: Remove `Session::dep_graph`

rustc: Remove `Session::dep_graph`

This commit removes the `dep_graph` field from the `Session` type according to
issue #44390. Most of the fallout here was relatively straightforward and the
`prepare_session_directory` function was rejiggered a bit to reuse the results
in the later-called `load_dep_graph` function.

Closes #44390