RFR of JDK-8158194: Rename the directory "InstalledModuleDescriptors" and any reference to "installed" to "system" in the tests

[Hamlin Li]

Would you mind to review the following patch?

bug: https://bugs.openjdk.java.net/browse/JDK-8158194
webrev: Rename the directory "InstalledModuleDescriptors" and any 
reference to "installed" to "system" in the tests

Renamed "InstalledModuleDescriptorPlugin" to 
"SystemModuleDescriptorPlugin" 
(jdk/tools/jlink/internal/plugins/SystemModuleDescriptorPlugin.java) 
before jigsaw was integrated to jdk9.
We also need to rename the directory "InstalledModuleDescriptors" 
(test/tools/jlink/plugins/InstalledModuleDescriptors) and any reference 
to "installed" to "system" in the tests

Thank you
-Hamlin