New "indexed" installable syntax: <drvPath>!<outputName>

Being conservative and only doing a single output name for now.

tests/build-explicit-output.sh

@@ -0,0 +1,17 @@
+source common.sh
+
+drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
+if nix build "$drv!not-an-output" --json; then
+    fail "'not-an-output' should fail to build"
+fi
+
+nix build "$drv!first" --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      .first and
+	  (has("second") | not)))
+'
+# TODO use
+#      (.first | match(".*multiple-outputs-a-first")) and
+# once we make it put the result paths in the buildables.