Embed p2 lockfiles for Eclipse JDT

[nedtwigg]

Starting with plugin-gradle 6.17.0 and plugin-maven 2.35.0, all our eclipse-based formatters migrated to use the equo p2 client. This means that they download their dependencies from the p2 repositories where the upstream Eclipse projects publish their jars. The downside is that it doesn't use the normal maven caching & proxies

• Maven and Gradle proxy settings not respected for Eclipse[Jdt/Cdt/Groovy] (equo-based steps) #1658
• Implement a global non-maven download policy #1669
• Move equo caches into build / build tool directory #1687

Relative to other ecosystems, the Eclipse steps have needed a lot more maintenance and had a lot less contributor interest.

We used to solve the p2 -> maven gap by bundling fat jars. Eventually we lost contributor interest to keep that maintained, so we switched to the equo p2 client, which lets us stay up to date with less maintenance burden.

For EclipseJDT, the jars are actually available on maven, we wouldn't have to use the p2 client if we brought back the lockfiles, and with Equo we could make those lockfiles in a more repeatable, easy to maintain way. That would bring at least Eclipse JDT fully back into maven world. If Eclipse CDT or Groovy ever get on board with maven, we could use the same approach there too.

PRs welcome :)

[BrotherPatrix]

"The downside is that it doesn't use the normal maven caching & proxies"
This doesn't help with offline builds on restrictive networks. We are not allowed to use internet connection, we had to copy Maven dependencies to a Nexus repository also closed from internet and we managed to get it working after a lot of paperwork, audits and antivirus scans, but because of this plugin, trying to download external dependencies is very difficult because we can't access internet. At least give us the options and documentation to provide these libraries manually, because this doesn't help me at all, at least not me.

[xjtushilei]

Our network is not very good, I hope we can improve this problem.

cc @hydrogen666

[obfischer]

We have the same problem as @BrotherPatrix in my current project. So it would be nice to be able to download all required dependencies via Maven.