[FIX] web, base: review lazy loaded <script> definitions

The assets bundle system allows to generate `<script>` elements whose
resource is meant to be lazy loaded. In that case, a "data-src" is used
instead of "src" directly as attribute. This is valid HTML code: "src"
is not required. However... this lazy loading system also automatically
added `defer="defer"` which makes the "src" attribute required, thus
failing W3C validation.

Now, we only add "data-src" and the `defer="defer"` part is added on the
client-side, once the "data-src" is switched to "src". Note that this
"defer" attribute may not be needed in this case at all, but it cannot
hurt.

Part-of: odoo/odoo#120311

addons/web/static/src/legacy/js/public/lazyloader.js

@@ -103,6 +103,7 @@ function _loadScripts(scripts, index) {
     }
     var script = scripts[index];
     script.addEventListener('load', _loadScripts.bind(this, scripts, index + 1));
+    script.setAttribute('defer', 'defer');
     script.src = script.dataset.src;
     script.removeAttribute('data-src');
 }

odoo/addons/base/models/assetsbundle.py

@@ -191,7 +191,9 @@ class AssetsBundle(object):
             src = self.get_debug_asset_url(name=js_attachment.name, extension='') if is_debug_assets else js_attachment[0].url
             attr = dict([
                 ["async", "async" if async_load else None],
-                ["defer", "defer" if defer_load or lazy_load else None],
+                # lazy_load will add defer in JS otherwise this is not W3C valid
+                # (defer is probably not needed there anyways)
+                ["defer", "defer" if defer_load else None],
                 ["type", "text/javascript"],
                 ["data-src" if lazy_load else "src", src],
                 ['data-asset-bundle', self.name],

odoo/addons/base/models/ir_qweb.py

@@ -2512,7 +2512,9 @@ class IrQWeb(models.AbstractModel):
                         "type": mimetype,
                     }
                     attributes["data-src" if lazy_load else "src"] = path
-                    if defer_load or lazy_load:
+                    if defer_load:
+                        # lazy_load will add defer in JS otherwise this is not
+                        # W3C valid (defer is probably not needed there anyways)
                         attributes["defer"] = "defer"
                 elif is_css:
                     tag = 'link'