Remove the restriction of directory based modules for runtime image linking

JDK-8311302 implements a feature to support jlink without packaged modules being present. Some of the jlink tests which use directory based modules fail with such a JDK since that clashes with the way linkable runtimes have been implemented (using JRTFS). We should investigate if that restriction is strictly necessary and remove it if possible.

The following tests have been marked as requiring packaged modules since they use directory based modules under the hood:

tools/jlink/JLink100Modules.java
tools/jlink/JLinkDedupTestBatchSizeOne.java
tools/jlink/JLinkReproducibleTest.java
tools/jlink/multireleasejar/JLinkMRJavaBaseVersionTest.java

These two tests require packaged modules, because of the way the Helper class is implemented (expecting packaged modules to be present):

tools/jlink/plugins/GenerateJLIClassesPluginTest.java
tools/jlink/plugins/IncludeLocalesPluginTest.java