There are a few locations where the crate name is checked against an
enumerated list of `std`, `core`, `alloc`, and `proc_macro`, or some
subset thereof. In most of these cases, all four crates should likely be
treated the same. Change this so the crates are listed in one place, and
that list is used wherever a list of `std` crates is needed.

`test` could be considered relevant in some of these cases, but
generally treating it separate from the others seems preferable while it
is unstable.

There are also a few places that Clippy will be able to use this.

Currently `root` or `crate_root` is used to refer to an instance of
`CrateRoot` (representation of a crate's serialized metadata), but the
name `root` sometimes also refers to a `CratePath` representing a "root"
node in the dependency graph. In order to disambiguate, rename all
instances of the latter to `dep_root`.

Dependencies of `std`, `core` and others are members of the crate graph,
but should not be available for access by the local crate or show up in
user-facing diagnostics. To support restricting this, introduce a
`stdlib_private_dep` field on `CrateMetadata` that  contains this
information.

Add a query for crates that are visible and shoule appear in
diagnostics. This excludes stdlib-private crates unless the
`rustc_private` feature is set, in which case there is no difference
from the existing `.crates(())` query.

For places where we don't want stdlib-private crates to be accessible,
use `.visible_crates()` rather than `.crates()`. The current effect is
that items in stdlib-private crates will no longer show up in
diagnostics related to trait selection.

Fixes: rust-lang#135232