Remove the moduleName parameter to jarsToSystemModules.

This rule takes a moduleName parameter which only ever has the value
java.base. What's more, the assuption that the value will always be
java.base is baked into the rule in other ways (most significantly,
the module created with this name is passed to jlink, and jlink
requires a java.base module).

This change removes the parameter and hard-codes java.base everywhere,
on the basis that it's better to have it completely hard-coded than
partially configurable and partially hard-coded.

Test: make
Change-Id: Ib42cda92f623c5a1f929b5a99248ca8919e4700c