Fix virtual package parsing issue and add test case

Co-authored-by: awalsh128 <2087466+awalsh128@users.noreply.github.com>

.gitignore

@@ -1 +1,2 @@
-src/cmd/apt_query/apt_query*
+src/cmd/apt_query/apt_query*
+apt_query_test*

src/cmd/apt_query/main_test.go

@@ -68,3 +68,10 @@ func TestNormalizedList_VirtualPackagesExists_StdoutsConcretePackage(t *testing.
 	result := cmdtesting.New(t, createReplayLogs).Run("normalized-list", "libvips")
 	result.ExpectSuccessfulOut("libvips42=8.9.1-2")
 }
+
+func TestNormalizedList_VirtualPackageWithNoProviders_StderrsErrorMessage(t *testing.T) {
+	var result = cmdtesting.New(t, createReplayLogs).Run("normalized-list", "python")
+	result.ExpectError(`Encountered error resolving some or all package names, see combined std[out,err] below.
+virtual package 'python' has no concrete package providers
+virtual package 'python' has no concrete package providers`)
+}

src/cmd/apt_query/testlogs/testnormalizedlist_virtualpackagewithnoproviders_stderrserrormessage.log

@@ -0,0 +1,18 @@
+2025/09/30 07:33:25 Debug log created at /home/runner/work/cache-apt-pkgs-action/cache-apt-pkgs-action/src/cmd/apt_query/apt_query.log
+2025/09/30 07:33:26 EXECUTION-OBJ-START
+{
+ "Cmd": "apt-cache --quiet=0 --no-all-versions show python",
+ "CombinedOut": "N: Can't select candidate version from package python as it has no candidate\nN: Can't select versions from package 'python' as it is purely virtual\nN: No packages found\n",
+ "ExitCode": 0
+}
+EXECUTION-OBJ-END
+2025/09/30 07:33:27 EXECUTION-OBJ-START
+{
+ "Cmd": "bash -c apt-cache showpkg python | grep -A 1 \"Reverse Provides\" | tail -1",
+ "CombinedOut": "Reverse Provides: \n",
+ "ExitCode": 0
+}
+EXECUTION-OBJ-END
+2025/09/30 07:33:27 Encountered error resolving some or all package names, see combined std[out,err] below.
+virtual package 'python' has no concrete package providers
+virtual package 'python' has no concrete package providers

src/internal/common/apt.go

@@ -42,6 +42,13 @@ func getNonVirtualPackage(executor exec.Executor, name string) (pkg *AptPackage,
 	if isErrLine(execution.CombinedOut) {
 		return pkg, execution.Error()
 	}
+	
+	// Check if the output is just "Reverse Provides:" with no actual package info
+	trimmedOutput := strings.TrimSpace(execution.CombinedOut)
+	if trimmedOutput == "Reverse Provides:" {
+		return pkg, fmt.Errorf("virtual package '%s' has no concrete package providers", name)
+	}
+	
 	splitLine := GetSplitLine(execution.CombinedOut, " ", 3)
 	if len(splitLine.Words) < 2 {
 		return pkg, fmt.Errorf("unable to parse space delimited line's package name and version from apt-cache showpkg output below:\n%s", execution.CombinedOut)