What does this PR try to resolve?

EnvConfigValue was wrapped in Value<T>,
which captured the table-level Definition for path resolution.

1. In merge_helper,
   we merge all keys in a table but not the definition of the table itself:
   

   cargo/src/cargo/util/context/config_value.rs

   Lines 177 to 199 in 60960cb

   	(&mut CV::Table(ref mut old, _), CV::Table(ref mut new, _)) => {
   	for (key, value) in mem::take(new) {
   	match old.entry(key.clone()) {
   	Entry::Occupied(mut entry) => {
   	let new_def = value.definition().clone();
   	let entry = entry.get_mut();
   	parts.push(&key);
   	entry.merge_helper(value, force, parts).with_context(|| {
   	format!(
   	"failed to merge key `{}` between \
   	{} and {}",
   	key,
   	entry.definition(),
   	new_def,
   	)
   	})?;
   	}
   	Entry::Vacant(entry) => {
   	entry.insert(value);
   	}
   	};
   	}
   	}

   It is probably hard to defined the source definition of a merged struct though, perhaps people shouldn't access it to do anything meaningful.

2. When deserializing a [env] struct with Value<T>,
   Value<T> delegate to ValueDeserializer to deserialize the value,
   which only takes the struct definition into account.
   And that turns out to be the stale table definition:
   

   cargo/src/cargo/util/context/de.rs

   Lines 119 to 137 in 60960cb

   	fn deserialize_struct<V>(
   	self,
   	name: &'static str,
   	fields: &'static [&'static str],
   	visitor: V,
   	) -> Result<V::Value, Self::Error>
   	where
   	V: de::Visitor<'de>,
   	{
   	// Match on the magical struct name/field names that are passed in to
   	// detect when we're deserializing `Value<T>`.
   	//
   	// See more comments in `value.rs` for the protocol used here.
   	if name == value::NAME && fields == value::FIELDS {
   	let source = ValueSource::with_deserializer(self)?;
   	return visitor.visit_map(ValueDeserializer::new(source));
   	}
   	visitor.visit_map(ConfigMapAccess::new_struct(self, fields)?)
   	}

3. Unlike file config discovery,
   include is loaded before the including config,
   hence its definition is stale and is used.

This caused in #16954 that [env] struct got a stale old config source definition.

How to test and review this PR?

New regression tests should have covered cases in #16954, and also captured the non-canonicalized case which is a separate issue.

This fix doesn't touch the core problem that a merged Value<T> table holds a stale source definition. Holding either new or old doesn't seem correct to me, as it is a merged one. This is really a footgun.

Fixes #16954