Run feature tests only on purl crate

Using `--no-default-features` on the workspace does not work as expected
because the `purl_test` and `xtask` crates depend on certain features of
the `purl` crate. So with feature unification, the `purl` crate is built
with those features **enabled**.

To resolve this and simplify the tests, this patch switches the defualt
and minimal tests to only test the `purl` crate itself. The "Full" test
is left to test the entire workspace with code coverage.

This technically leaves open the possibility that `purl_test` or `xtask`
may not successfully build with default features... But since those are
test and development crates, I'm not actually concerned.

.github/workflows/main.yml

@@ -50,13 +50,13 @@ jobs:
         include:
           - name: Minimal
             runner: test
-            features: --no-default-features
+            args: -p purl --no-default-features
           - name: Default
             runner: test
-            features: ""
+            args: -p purl
           - name: Full
             runner: tarpaulin
-            features: --all-features
+            args: --all-features
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
@@ -73,4 +73,4 @@ jobs:
         rm tarpaulin.tar.gz
 
     - name: Run tests
-      run: cargo ${{ matrix.runner }} ${{ matrix.features }}
+      run: cargo ${{ matrix.runner }} ${{ matrix.args }}