Add tests of visibility of private direct submodules (#6849)

## Description

#5765 was solved by an earlier PR, but we don't seem to have regression
tests for that situation. This PR adds those tests.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Author: jjcnn

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/Forc.lock

@@ -0,0 +1,3 @@
+[[package]]
+name = "submodule_visibility"
+source = "member"

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/Forc.toml

@@ -0,0 +1,6 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "submodule_visibility"
+implicit-std = false

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/src/main.sw

@@ -0,0 +1,16 @@
+library;
+
+// other is a submodule that declares a private submodule lib
+// lib contains a declaration of the public struct S, but since lib is private it is not visible here.
+// It is visible inside other, though.
+pub mod other;
+
+// lib is private, and not a direct submodule of the current module, so this should fail
+use other::lib::S; 
+
+pub fn foo() {
+    let my_struct = S { val: 0 };
+
+    // lib is private, and not a direct submodule of the current module, so this should fail
+    let my_other_struct = lib::T { val: 1 };
+}

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/src/other.sw

@@ -0,0 +1,11 @@
+library;
+
+mod lib; // private submodule
+
+// lib is private, but since it is a direct submodule we can access its public items.
+use lib::S;
+
+// Public function
+pub fn foo() {
+    let my_struct = S { val: 0 };
+}

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/src/other/lib.sw

@@ -0,0 +1,9 @@
+library;
+
+pub struct S {
+    pub val: u64,
+}
+
+pub struct T {
+    pub val: u64,
+}

test/src/e2e_vm_tests/test_programs/should_fail/submodule_visibility/test.toml

@@ -0,0 +1,9 @@
+category = "fail"
+
+# check: $()error
+# check: $()Module "lib" is private.
+
+# check: $()error
+# check: $()Module "lib" could not be found.
+
+# check: $()Aborting due to 2 errors.

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/Forc.lock

@@ -0,0 +1,3 @@
+[[package]]
+name = "submodule_visibility"
+source = "member"

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/Forc.toml

@@ -0,0 +1,6 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "submodule_visibility"
+implicit-std = false

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/src/main.sw

@@ -0,0 +1,14 @@
+library;
+
+// other is a submodule that declares a private submodule lib
+// lib contains a declaration of the public struct S, but since lib is private it is not visible here.
+// It is visible inside other, though.
+pub mod other;
+
+// other reexports other::lib::U, so we can access it as other::U
+use other::U;
+
+// Public function
+pub fn foo() {
+    let my_struct = U { val: 0 };
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/src/other.sw

@@ -0,0 +1,18 @@
+library;
+
+mod lib; // private submodule
+
+// lib is private, but since it is a direct submodule we can access its public items.
+use lib::S;
+
+// lib is private, but since it is a direct submodule we can access its public items.
+// Reexporting it makes it visible from here, but not from lib
+pub use lib::U;
+
+// Public function
+pub fn foo() {
+    let my_struct = S { val: 0 };
+
+    // lib is private, but since it is a direct submodule we can access its public items.
+    let my_other_struct = lib::T { val: 1 };
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/src/other/lib.sw

@@ -0,0 +1,13 @@
+library;
+
+pub struct S {
+    pub val: u64,
+}
+
+pub struct T {
+    pub val: u64,
+}
+
+pub struct U {
+    pub val: u64,
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/submodule_visibility/test.toml

@@ -0,0 +1,2 @@
+category = "compile"
+expected_warnings = 3