[Perf] Useless classpath expansion (reading MANIFEST.MF)

[mickaelistria]

While profiling, I noticed that Javac seems to automatically expand the classpath for the dependencies: it does read all the jar files, look for some MANIFEST.MF and tries to read a Class-Path element in it to expand it.
The following screnshot shows the stack causing that:

On that particular recording, this operation accounts for ~8% of the time when requesting completion.

This operation is useless in Eclipse IDE, which does already resolve the classpath. It would be interesting to get rid of it. This would probably involve overriding some parts in the JavacFileManager, but a dummy attempt has proven it to be non-trivial as the Locations class is only package-visible.

[mickaelistria]

An interesting entry point for debugging is SearchPath.expandJarClassPaths()