Allow use of Studio as a proxy-able Maven repository

Configuring https://connect.nuxeo.com/nuxeo/site/studio/maven as a Maven repository in the POM allows to retrieve artifacts from it, since Maven will concatenate the wanted artifact's GAV to the URL: https://connect.nuxeo.com/nuxeo/site/studio/maven/${groupId}/${artifactId}/${version}/... (with groupId=nuxeo-studio and artifactId=the studio project).
But it doesn't work if we want to set a Nexus proxy on it.
There are two possible causes:

• https://connect.nuxeo.com/nuxeo/site/studio/maven and https://connect.nuxeo.com/nuxeo/site/studio/maven/nuxeo-studio return 404 (so Nexus considers the repository is down or unavailable).
• available projects (artifactId) are not listed under https://connect.nuxeo.com/nuxeo/site/studio/maven/nuxeo-studio

1) https://connect.nuxeo.com/nuxeo/site/studio/maven must return nuxeo-studio/ which is the only available groupId under that URL
2) Maybe, https://connect.nuxeo.com/nuxeo/site/studio/maven/nuxeo-studio/ should list the available projects for the currently authenticated user.

Note Nexus should be able to work with the repository even if there's no index file at its root.