Rename index.js adjunct to avoid view-name collision

The new external CSP-compliant script was named `index.js` and pulled in
with `<st:adjunct includes="...RestListParameterDefinition.index"/>`.
Stapler resolves that include against the same `RestListParameterDefinition`
class and treats `.index` as a view, recursively rendering `index.jelly`
with the Adjunct object as `it`. The build form ends up with two
`<select name="value">` (the second one with an empty `id="-select"`),
which the browser submits as two `value` fields and produces a 500 on
the server.

Renaming the file to `selectInit.js` and updating the adjunct include
removes the collision with the existing view name; only one parameter
widget is rendered and submit works again.

Verified on jenkins.sandbox.asatnet.net: single dropdown with the empty
"—" entry, empty value submits as an empty string, the build succeeds.

Author: Arbuzov

src/main/resources/io/jenkins/plugins/restlistparam/RestListParameterDefinition/index.jelly

@@ -6,7 +6,7 @@
 
   <st:adjunct includes="io.jenkins.plugins.jquery3"/>
   <st:adjunct includes="io.jenkins.plugins.select2"/>
-  <st:adjunct includes="io.jenkins.plugins.restlistparam.RestListParameterDefinition.index"/>
+  <st:adjunct includes="io.jenkins.plugins.restlistparam.RestListParameterDefinition.selectInit"/>
   <j:set var="id" value="${h.generateId()}"/>
 
   <link rel="stylesheet" type="text/css" href="${resURL}/plugin/rest-list-parameter/css/select2-bootstrap4.min.css"/>