fix(config): normalize included config paths (#16964)

### What does this PR try to resolve?

Extracted from <https://github.com/rust-lang/cargo/pull/16957>.

Without normalizing included config paths,
`Definition::root()` will do `parent().parent()`
on non-normalized paths containing `..` segments.
And that will remove `..` and result in wrong path resolution.

### How to test and review this PR?

Two tested are added to showcase the buggy behavior, especially
`env_relative_path_included_from_upper_level` which was a wrong path
resolution.

src/cargo/util/context/mod.rs

@@ -2347,6 +2347,7 @@ impl ConfigInclude {
             Definition::Environment(_) | Definition::Cli(None) | Definition::BuiltIn => gctx.cwd(),
         }
         .join(&self.path);
+        let abs_path = paths::normalize_path(&abs_path);
 
         if self.optional && !abs_path.exists() {
             tracing::info!(

tests/testsuite/config_include.rs

@@ -1,6 +1,7 @@
 //! Tests for `include` config field.
 
 use crate::prelude::*;
+use cargo_test_support::compare::assert_e2e;
 use cargo_test_support::str;
 
 use super::config::GlobalContextBuilder;
@@ -587,3 +588,60 @@ Caused by:
 "#]],
     );
 }
+
+#[cargo_test]
+fn env_relative_path_included_from_same_level() {
+    // See https://github.com/rust-lang/cargo/issues/16954
+    write_config_at(
+        "foo/.cargo/config.toml",
+        "
+        include = ['../../inc/inc.toml']
+
+        [env]
+        INNER = { value = 'inner-val', relative = true }
+        ",
+    );
+    write_config_at(
+        "inc/inc.toml",
+        "
+        [env]
+        OUTER = { value = 'outer-val', relative = true }
+        ",
+    );
+    let gctx = GlobalContextBuilder::new().cwd("foo").build();
+    let env = gctx.env_config().unwrap();
+
+    assert_e2e().eq(
+        env.get("INNER").unwrap().to_str().unwrap(),
+        str!["[ROOT]/foo/inner-val"],
+    );
+    assert_e2e().eq(
+        env.get("OUTER").unwrap().to_str().unwrap(),
+        str!["[ROOT]/outer-val"],
+    );
+}
+
+#[cargo_test]
+fn env_relative_path_included_from_upper_level() {
+    // See https://github.com/rust-lang/cargo/issues/16954
+    write_config_at(
+        "outer/foo/.cargo/config.toml",
+        "
+        include = ['../../inc.toml']
+        ",
+    );
+    write_config_at(
+        "outer/inc.toml",
+        "
+        [env]
+        MY_ENV = { value = 'val', relative = true }
+        ",
+    );
+    let gctx = GlobalContextBuilder::new().cwd("outer/foo").build();
+    let env = gctx.env_config().unwrap();
+
+    assert_e2e().eq(
+        env.get("MY_ENV").unwrap().to_str().unwrap(),
+        str!["[ROOT]/val"],
+    );
+}