Implement support for registry packages.

This commit implements support for referencing packages from a registry.

A new `wac-resolver` crate was added and the resolvers implementation was
refactored into it; package resolution now occurs before document resolution.

A `registry` feature was added to the CLI tool; when enabled, a `--registry`
CLI option is accepted to point at the registry to use.

The CLI also supports Warg client configuration files (default or local).

A simple `indicatif` progress bar was added to display the number of packages
that need to be downloaded from the registry.

A new integration test that uses packages from a registry was also implemented.

Author: peterhuene

.github/workflows/main.yml

@@ -25,6 +25,30 @@ jobs:
     - name: Run all tests
       run: cargo test --all
 
+  check:
+    name: Check feature combinations
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install Rust
+      run: |
+        rustup update stable --no-self-update
+        rustup default stable
+      shell: bash
+    - name: Build without default features
+      run: cargo check --no-default-features
+    - name: Build the `wat` feature
+      run: cargo check --no-default-features --features wat
+    - name: Build the `wit` feature
+      run: cargo check --no-default-features --features wit
+    - name: Build the `registry` feature
+      run: cargo check --no-default-features --features registry
+    - name: Build all features
+      run: cargo check --all-features
+
   rustfmt:
     name: Format source code
     runs-on: ubuntu-latest