P2Export: Add repository-references to content.xml

[chrisrueger]

The current P2Export creates a self contained P2 repo jar.
A problem came up recently with regards to ECF functionality.

The installation of the "Bndtools ECF remote services" failed

Cannot complete the install because one or more required items could not be found.
  Software being installed: Bndtools ECF Remote Services 7.2.0.DEV-202508271152-gccb11de (bndtools.ecf.feature.feature.group 7.2.0.DEV-202508271152-gccb11de)
  Missing requirement: Bndtools ECF Remote Services 7.2.0.DEV-202508271152-gccb11de (bndtools.ecf.feature.feature.group 7.2.0.DEV-202508271152-gccb11de) requires 'org.eclipse.equinox.p2.iu; org.eclipse.ecf.remoteservice.sdk.bndtools.feature.feature.group 0.0.0' but it could not be found

The solution currently was to first add the ECF update site to Eclipse.

• https://download.eclipse.org/rt/ecf/latest/site.p2/

After that the installation worked fine.

Repository references

P2 has a repository-reference feature, so that this update site URL can be made available in the P2 repo itself.

If we add that then installation gets easier so that the user does not need to know and add the ECF update site upfront.

This is needed by #6777

[chrisrueger]

Ha seems the P2Export supports this already, but I have not noticed it.
https://bnd.bndtools.org/plugins/p2export.html

The update URL and label can also be specified with macros:

update          =   https://bndtools.jfrog.io/bndtools/update-latest
update.label    =   Bndtools Update Site

But it is not sufficient, since it does not support an update site url per feature.
I will add that

[chrisrueger]

Closed via #6789