Switch plugin packages back to git submodules (#678)

Alright, so I hate to be the one to do this, but #584 broke crates.io
publishing and also caused librojo to be unusable. I see that there was
some discussion on Discord shortly after the problem was realized, but
there was no action taken.

I think keeping librojo and publishing working far, far outweigh any
convenience added by Wally.

I've kept the same `Packages` naming convention to keep the diff
minimal.

.github/workflows/ci.yml

@@ -20,6 +20,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+        submodules: true
 
     - name: Install Rust
       uses: actions-rs/toolchain@v1
@@ -33,12 +35,6 @@ jobs:
       with:
         version: 'v0.2.7'
 
-    - name: Install packages
-      run: |
-        cd plugin
-        wally install
-        cd ..
-
     - name: Build
       run: cargo build --locked --verbose
 
@@ -51,6 +47,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+        submodules: true
 
     - name: Install Rust
       uses: actions-rs/toolchain@v1
@@ -64,14 +62,8 @@ jobs:
       with:
         version: 'v0.2.7'
 
-    - name: Install packages
-      run: |
-        cd plugin
-        wally install
-        cd ..
-
     - name: Rustfmt
       run: cargo fmt -- --check
 
     - name: Clippy
-      run: cargo clippy
+      run: cargo clippy