This is phase 1 of getting sun.nio.cs (java.base) and sun.nio.cs.ext (jdk.charsets) to work with module system.
The current module system implementation requires the charset that might be used during vm startup to be in
java.base module. The charset build mechanism needs to be able to configure a set of charsets that currently
in extended charsets pakcage (sun.nio.cs.ext) to be re-located into sun.nio.cs during build time.
GB18030 and EUC_TW, which both have a "huge' mapping table, will be left for the next phase. These two might
need to be re-implemented to fit into the java.base/sun.nio.cs