[DRAFT] An idea how to restructure JDT core to get proper ecj bundle

The org.eclipse.jdt.core.ecj.validation was a "dummy" bundle so far,
used only to validate compilation issues in IDE.

That one should be renamed (org.eclipse.jdt.core.ecj.validation ->
org.eclipse.jdt.core.compiler.batch) and be a proper maven library.

It is actually the ecj compiler library without any dependencies, that
could be consumed by JDT and the rest of the world.

It must be required and re-exported by JDT core.
Unfortunately, there are two split packages:

org.eclipse.jdt.internal.compiler
org.eclipse.jdt.internal.compiler.parser

Not sure what's the best way to deal with it, may be rename those
smaller packages in in jdt.core to avoid split packages issue, they are
internal, so it shouldn't break many clients (but most likely will, like
Xtext), but I haven't tried it and don't know if they use package
protected API.

org.eclipse.jdt.compiler.apt and org.eclipse.jdt.compiler.tool were
fragments of jdt.core, now they would be inside
org.eclipse.jdt.core.compiler.batch.

TODO:

1) What I did NOT tried is to re-write all the magic scripts that build
and package separated ecj library out of jdt.core.
2) The 3 antadapter classes are now split over ecj and jdt core, because
BuildJarIndex.java and CheckDebugAttributes.java depend on JDT core
code.
3) ecj.1 and build_ecj.xml aren't touched yet. ecj.1 was touched last
time 2017, it seem to be used for man pages.
4) pom from jdt core will need an adoption.
5) org.eclipse.jdt-feature need to be updated
6) TBC

See
- eclipse-jdt#181
- eclipse-platform/eclipse.platform.ua#18

org.eclipse.jdt.compiler.apt.tests/META-INF/MANIFEST.MF

@@ -7,9 +7,10 @@ Bundle-Vendor: %providerName
 Bundle-Localization: plugin
 Bundle-RequiredExecutionEnvironment: JavaSE-11
 Require-Bundle: org.junit,
- org.eclipse.jdt.core;bundle-version="3.27.0",
+ org.eclipse.jdt.core;bundle-version="3.32.0",
  org.eclipse.core.resources,
- org.eclipse.core.runtime
+ org.eclipse.core.runtime,
+ org.eclipse.jdt.core.compiler.batch;bundle-version="3.11.0"
 Export-Package: org.eclipse.jdt.compiler.apt.tests,
  org.eclipse.jdt.compiler.apt.tests.annotations,
  org.eclipse.jdt.compiler.apt.tests.processors.base,