quadlet: Support [Install] for templated units

For a base template like `foo@.container` the WantedBy and RequiredBy
keys does nothing. However, if a DefaultInstance= key is specified
that is used by default.

However, even if the DefaultInstance= is not given, the Install
section is still useful, because you can instantiate the generic
template by making a symlink for it, and that symlink will then
pick up the instance id.

So, for example, this foo@.container will not enable anything
on boot.
```
[Container]
Image=foo
Exec=sleep 100

[Install]
WantedBy=other.container
```

But if you have a symlink 'foo@instance.container` -> `foo@.container'
then the `foo@instance` service will be marked as wanted by `other`.

In addition, even if the main template doesn't have an Install
section, you can instantiate it with a symlink like above, and then
enabling it using a dropin file like
foo@instance.container.d/install.conf containing:
```
[Install]
WantedBy=other.container
```

Signed-off-by: Alexander Larsson <alexl@redhat.com>

cmd/quadlet/main.go

@@ -345,11 +345,27 @@ func enableServiceFile(outputPath string, service *parser.UnitFile) {
 		symlinks = append(symlinks, filepath.Clean(alias))
 	}
 
+	serviceFilename := service.Filename
+	templateBase, templateInstance := service.GetTemplateParts()
+
+	// For non-instantiated template service we only support installs if a
+	// DefaultInstance is given. Otherwise we ignore the Install group, but
+	// it is still useful when instantiating the unit via a symlink.
+	if templateBase != "" && templateInstance == "" {
+		if defaultInstance, ok := service.Lookup(quadlet.InstallGroup, "DefaultInstance"); ok {
+			parts := strings.SplitN(templateBase, "@", 2)
+			serviceFilename = parts[0] + "@" + defaultInstance + parts[1]
+		} else {
+			serviceFilename = ""
+		}
+	}
+
+	if serviceFilename != "" {
 		wantedBy := service.LookupAllStrv(quadlet.InstallGroup, "WantedBy")
 		for _, wantedByUnit := range wantedBy {
 			// Only allow filenames, not paths
 			if !strings.Contains(wantedByUnit, "/") {
-			symlinks = append(symlinks, fmt.Sprintf("%s.wants/%s", wantedByUnit, service.Filename))
+				symlinks = append(symlinks, fmt.Sprintf("%s.wants/%s", wantedByUnit, serviceFilename))
 			}
 		}
 
@@ -357,7 +373,8 @@ func enableServiceFile(outputPath string, service *parser.UnitFile) {
 		for _, requiredByUnit := range requiredBy {
 			// Only allow filenames, not paths
 			if !strings.Contains(requiredByUnit, "/") {
-			symlinks = append(symlinks, fmt.Sprintf("%s.requires/%s", requiredByUnit, service.Filename))
+				symlinks = append(symlinks, fmt.Sprintf("%s.requires/%s", requiredByUnit, serviceFilename))
+			}
 		}
 	}