RFR: 8214230: Classes generated by SystemModulesPlugin.java are not reproducable

[Erik Helin]

Hi all,

this small patch ensures that two classes generated by jlink's 
SystemModulePlugin (SystemModules$default and SystemModules$all) are 
generated in a reproducible manner. That is, if you run jlink two times 
in a row with identical input, then the generated classes from both runs 
should be identical. This was almost the case already, the only issue is 
that some code in SystemModulesPlugin.java are iterating over Sets, and 
Set does not have a specified iteration order (the iteration order did 
differ between runs).

This patch ensures that the iteration order over the sets is 
well-defined by creating an ArrayList from the Set, sorting that 
ArrayList and then finally iterate over the sorted ArrayList instead of 
the Set.

For smaller programs jlink will now produce reproducible results, the 
entire lib/modules file will be reproducible [0]. I have added a JTReg 
test to verify this property (again, for smaller programs).

Webrev:
https://cr.openjdk.java.net/~ehelin/8214230/00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8214230

Testing:
- tier1, tier2 and tier3 on macOS, Windows, Linux (all x86-64).
- Run newly developed JTReg test on macOS, Windows, Linux (all x86-64).
- Run newly developed JTReg test with the fix absent and verified that
   the test failed.

Many thanks to Alan Bateman for helping out with the patch!

Thanks,
Erik

[0]: https://reproducible-builds.org/docs/definition/