nix-env: add --outputs flag

This isn't particularly useful since the json output already includes
metadata.  However the evaluation behaves differently, in this case
`meta.outputsToInstall` is also validated.  The same is done by the
channel index generation but it's currently not exposed in a way other
tools like ofborg could use it.

https://github.com/NixOS/nixos-channel-scripts/blob/7a681103b2b3ce150f7f96394f19aa0ad4797ca1/generate-programs-index.cc#L110

doc/manual/command-ref/nix-env.xml

@@ -924,6 +924,7 @@ $ nix-env --set-flag priority 10 gcc</screen>
   <arg><option>--system</option></arg>
   <arg><option>--drv-path</option></arg>
   <arg><option>--out-path</option></arg>
+  <arg><option>--outputs</option></arg>
   <arg><option>--description</option></arg>
   <arg><option>--meta</option></arg>
 
@@ -1144,6 +1145,14 @@ user environment elements, etc. -->
 
   </varlistentry>
 
+  <varlistentry><term><option>--outputs</option></term>
+
+    <listitem><para>Print the outputs of the derivation.
+    Unlike <option>--out-path</option>, only the outputs that would be installed are
+    listed.</para></listitem>
+
+  </varlistentry>
+
   <varlistentry><term><option>--description</option></term>
 
     <listitem><para>Print a short (one-line) description of the

src/nix-env/nix-env.cc

@@ -894,6 +894,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
     bool printSystem = false;
     bool printDrvPath = false;
     bool printOutPath = false;
+    bool printOutputs = false;
     bool printDescription = false;
     bool printMeta = false;
     bool compareVersions = false;
@@ -913,6 +914,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         else if (arg == "--compare-versions" || arg == "-c") compareVersions = true;
         else if (arg == "--drv-path") printDrvPath = true;
         else if (arg == "--out-path") printOutPath = true;
+        else if (arg == "--outputs") printOutputs = true;
         else if (arg == "--meta") printMeta = true;
         else if (arg == "--installed") source = sInstalled;
         else if (arg == "--available" || arg == "-a") source = sAvailable;
@@ -1096,6 +1098,16 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                 columns.push_back(s);
             }
 
+            if (printOutputs && !xmlOutput) {
+                DrvInfo::Outputs outputs = i.queryOutputs(true);
+                string s;
+                for (auto & j : outputs) {
+                    if (!s.empty()) s += ';';
+                    s += j.first;
+                }
+                columns.push_back(s);
+            }
+
             if (printDescription) {
                 string descr = i.queryMetaString("description");
                 if (xmlOutput) {