Build breaks on invalid _referenced_ update site

[Bananeweizen]

Tycho 5.0 SNAPSHOT cannot build https://github.com/MoreUnit/MoreUnit-Eclipse/tree/master/org.moreunit.build (while Tycho 4 can). The target platform for MoreUnit contains the TestNG update site, which in turn contains a reference to the m2e update site, and the URL of that reference is not valid for P2:

...
[INFO] Downloading from p2: https://testng.org/testng-p2-update-site/7.7.1/p2.index
[INFO] Downloading from p2: https://testng.org/testng-p2-update-site/updatesites/7.7.1.r202305281608/p2.index
[INFO] Downloading from p2: https://testng.org/testng-p2-update-site/7.8.0/p2.index
[INFO] Downloading from p2: https://testng.org/testng-p2-update-site/updatesites/7.8.0.r202306071539/p2.index
[INFO] Downloading from p2: http://download.eclipse.org/technology/m2e/releases/p2.index
[INFO] Downloading from p2: http://download.eclipse.org/technology/m2e/releases/site.xml
...
[ERROR] Failed to resolve target definition file:/C:/dev/moreunit/git/MoreUnit-Eclipse/org.moreunit.core/../org.moreunit.build/eclipse-4.30.target: Failed to load p2 metadata repository from location http://download.eclipse.org/technology/m2e/releases/: No repository found at http://download.eclipse.org/technology/m2e/releases.

Trouble is: There is no way to exclude a referenced site from the target platform, AFAIK. I believe we should ignore failures that happen only in referenced sites (instead of included update sites).

Interestingly in the browser I get redirected to the https version of the update site, but with the P2 update site explorer of Oomph, neither the http nor https URL above are valid.

[laeubi]

@Bananeweizen does the site work when you use it directly in a target file? I think the p2 http>https redirect might be what makes it fail to load.