refactor(gctx): extract config schema to a module (#16195)

### What does this PR try to resolve?

Make config schema types a bit more outstanding from the way-too-long
`context/mod.rs` module.
The long-term(-and-hard-to-achieve) is to make a similar crate like
`cargo-util-schemas` for config, or just make these schemas into
`cargo-util-schemas`.

### How to test and review this PR?

This may conflict with #16194

Author: epage

src/cargo/core/compiler/compile_kind.rs

@@ -114,7 +114,7 @@ impl CompileKind {
         let kinds = match (fallback, &gctx.build_config()?.target) {
             (_, None) | (CompileKindFallback::JustHost, _) => Ok(vec![CompileKind::Host]),
             (CompileKindFallback::BuildConfig, Some(build_target_config)) => {
-                dedup(&build_target_config.values(gctx)?)
+                dedup(&build_target_config.values(gctx.cwd())?)
             }
         };
 

src/cargo/ops/resolve.rs

@@ -540,7 +540,7 @@ pub fn add_overrides<'a>(
         // The path listed next to the string is the config file in which the
         // key was located, so we want to pop off the `.cargo/config` component
         // to get the directory containing the `.cargo` folder.
-        (paths::normalize_path(&def.root(gctx).join(s)), def)
+        (paths::normalize_path(&def.root(gctx.cwd()).join(s)), def)
     });
 
     for (path, definition) in paths {