Stop MPC client from requesting known 404 on the marketplace website
There are many requests to invalid URLs on the marketplace such as:
```
/content/testng-eclipse/site.xml
/content/markdown-text-editor/artifacts.xml
/marketplace-client-intro/content.xml.xz?mpc_install=162307
/content/eclipse-pmd/content.xml
/content/eclipse-pmd/content.xml.xz
/content/eclipse-pmd/compositeContent.xml
/marketplace-client-intro/compositeContent.jar?mpc_install=4355801
/marketplace-client-intro/content.jar?mpc_install=2566102
/content/spring-tools-4-aka-spring-tool-suite-4/compositeArtifacts.jar
/content/sonarlint/p2.index
```
These URLs will never work and it's uneccessary to query them, I'd be great if we could avoid making those requests altogether.
issue