Add support for nested submodules to forc doc (FuelLabs#4333)

## Description
Sort of hacky, but this PR fixes the support for nested submodules. I've
opened FuelLabs#4371 as a follow up to fix this. It also fixes a bug I found
while working on this, which was merging modules of the same name, even
if they were in different folders.

[Screencast from 2023-03-28
17-44-07.webm](https://user-images.githubusercontent.com/57543709/228384162-f94249db-66d8-42dd-9c77-33a31b633a8b.webm)

## Checklist

- [x] I have linked to any relevant issues. Closes FuelLabs#3929 Closes FuelLabs#4370 
- [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] 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.

forc-plugins/forc-doc/src/doc.rs

@@ -20,7 +20,7 @@ use sway_core::{
 pub(crate) type Documentation = Vec<Document>;
 /// A finalized Document ready to be rendered. We want to retain all
 /// information including spans, fields on structs, variants on enums etc.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub(crate) struct Document {
     pub(crate) module_info: ModuleInfo,
     pub(crate) item_header: ItemHeader,
@@ -127,8 +127,7 @@ impl Document {
                 }
             }
         }
-        // if there is another submodule we need to go a level deeper
-        if let Some((_, submodule)) = typed_submodule.module.submodules.first() {
+        for (_, submodule) in &typed_submodule.module.submodules {
             Document::from_ty_submodule(
                 decl_engine,
                 submodule,
@@ -152,10 +151,10 @@ impl Renderable for Document {
     }
 }
 
-pub(crate) type ModulePrefix = String;
+pub(crate) type ModulePrefixes = Vec<String>;
 #[derive(Debug, Clone, Ord, PartialOrd, Eq, PartialEq)]
 pub(crate) struct ModuleInfo {
-    pub(crate) module_prefixes: Vec<ModulePrefix>,
+    pub(crate) module_prefixes: ModulePrefixes,
     pub(crate) attributes: Option<String>,
 }
 impl ModuleInfo {

forc-plugins/forc-doc/src/tests/data/nested_subdirs/Forc.lock

@@ -0,0 +1,8 @@
+[[package]]
+name = 'core'
+source = 'path+from-root-2E974E96B45CBA16'
+
+[[package]]
+name = 'nested_subdirs'
+source = 'member'
+dependencies = ['core']

forc-plugins/forc-doc/src/tests/data/nested_subdirs/Forc.toml

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <[email protected]>"]
+entry = "lib.sw"
+license = "Apache-2.0"
+name = "nested_subdirs"
+
+[dependencies]
+core = { path = "../../../../../../sway-lib-core" }

forc-plugins/forc-doc/src/tests/data/nested_subdirs/src/lib.sw

@@ -0,0 +1,3 @@
+library;
+
+mod sub_dir;

forc-plugins/forc-doc/src/tests/data/nested_subdirs/src/sub_dir.sw

@@ -0,0 +1,3 @@
+library;
+
+mod folder;

forc-plugins/forc-doc/src/tests/data/nested_subdirs/src/sub_dir/folder.sw

@@ -0,0 +1,4 @@
+library;
+
+mod lib_one;
+mod lib_two;

forc-plugins/forc-doc/src/tests/data/nested_subdirs/src/sub_dir/folder/lib_one.sw

@@ -0,0 +1,5 @@
+library;
+
+pub struct Foo {
+    foo: u32,
+}

forc-plugins/forc-doc/src/tests/data/nested_subdirs/src/sub_dir/folder/lib_two.sw

@@ -0,0 +1,5 @@
+library;
+
+pub enum Bar {
+    bar: u32,
+}