Systematic maintenance of jclMinX.jar and .src.zip

[stephan-herrmann]

In https://bugs.eclipse.org/560658 we started to organize maintenance of jcl stubs more systematically, but covered only convertJclMin, not regular jclMin.

Most recently, follow-ups of https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2551 would like to work on a cleaned-up foundation :)

To consider:

• do we actually need two flavors, or can one set of jclMin cover all use cases?
• are all source projects configured for IDE and build using org.eclipse.jdt.core.tests.model/JCL/build.xml?
• 2712 brought up the issue of JCL_SRCROOT
  • this is used to configure classpath entries with a src path inside the src.zip archive
  • it will be used by all tests accessing sources of binary types from jclMin (example: BufferTests.testAppendReadOnly())
  • as per AbstractJavaModelTests.getExternalJCLRootSourcePath() paths inside src.zip should start with "src"
  • if this is misconfigured, tests may still pass, but require analysing jar and zip to tediously figure out the source root path