RFE: Better error message when JDK is detected as JRE due to missing jmods

[space88man]

Background: #530

RHEL/CentOS/Fedora distribute jmod files in a separate package java-11-openjdk-jmods which user may inadvertently not install. STS4 requires the jmod files to function optimally and shows the cryptic message:

The JRE you are running Eclipse with appears to not be a JDK.

In RHEL/CentOS/Fedora this was caused by not installing the jmods package.

Perhaps a better error message could be displayed like "Missing jmod files".

This has bugged me for months as STS4 would run fine with 3rd party JDK11s like AdoptOpenJDK/Zulu but not with the distro packaged JDK11.

[space88man]

@martinlippert has explained that jmod files are not used at runtime; the .exists() test on jmods/jdk.management.jmod et al. is a detection mechanism for JDK 9+.

Linux distros which don't install jmod files by default will now be a covered by a test for lib/libmanagement.so instead.

[martinlippert]

Actually, after investigating this in more detail on a various platforms, it looks to me like it is not easy to identify from the outside which modules are around inside of a specific JDK (>= 9). Therefore we changed the implementation to do the specific check for the tools.jar on JDK8 only and skip this for JDK >= 9. Therefore, I think, we can close this issue.