Auto merge of rust-lang#12971 - epage:schema, r=Muscraft

refactor(toml): Clean up workspace inheritance

### What does this PR try to resolve?

The goal is to simplify the code so we have a better boundary between `toml/mod.rs` and `toml/schema.rs` for when we break `toml/schema.rs` into a separate package for rust-lang#12801.

This let us
- Remove a trait used in some back and forth for error handling
- Move a lot of the inheritable bookkeeping and logic out of `schema.rs`

A lot of these changes were inspired by [`cargo_toml`](https://docs.rs/cargo_toml/latest/cargo_toml/).  This included some renaming that I felt made the code clearer.

I didn't go as far as `cargo_toml`, yet.
- They derive more `Deserializers`, producing worse errors
- Their equivalent of `InheritableField` has an `inherit` function on it.  They eagerly inherit things and then error if anything isn't inherited
  - I'm still toying with something like this but held off for now
  - One idea is `InheritableField` has an `inherit_with` function, like today, that only passes errors up but doesn't generate an error.  We then have a `get` function that errors if it isn't inherited.  We could encode the field names, for error reporting, into a type parameter for `InheritableField`
- They flatten `InheritableDependency` into `TomlDependency`
  - By splitting these up, `workspace.dependencies` and `.cargo/config.toml` code can directly use `TomlDependency` without extra error handling
  -If we went this rout, I think I'd merge`InheritableDependency::Inherit` with `DetailedDependency`, having callers handle the errors (much like `TomlManifest` is both a real and virtual)

Some things I'm trying to balance
- Complexity
- Quality of error messages
- Knowing what code needs touching when changes are made
  - Some more work is needed here

### How should we test and review this PR?

### Additional information

src/cargo/core/mod.rs

@@ -14,7 +14,6 @@ pub use self::workspace::{
     find_workspace_root, resolve_relative_path, MaybePackage, Workspace, WorkspaceConfig,
     WorkspaceRootConfig,
 };
-pub use crate::util::toml::schema::InheritableFields;
 
 pub mod compiler;
 pub mod dependency;

src/cargo/core/workspace.rs

@@ -23,7 +23,7 @@ use crate::util::edit_distance;
 use crate::util::errors::{CargoResult, ManifestError};
 use crate::util::interning::InternedString;
 use crate::util::toml::{
-    read_manifest, schema::InheritableFields, schema::TomlDependency, schema::TomlProfiles,
+    read_manifest, schema::TomlDependency, schema::TomlProfiles, InheritableFields,
 };
 use crate::util::RustVersion;
 use crate::util::{config::ConfigRelativePath, Config, Filesystem, IntoUrl};