RFR: 8310379: Relax prerequisites for java.base-jmod target

[Erik Joelsson]

The top level target "java.base-jmod" currently has all other jmod targets on the prerequisites list. This is because we store a checksum for every non upgradeable module in java.base and most of the modules aren't upgradeable. But, since we do have upgradeable modules, those shouldn't be on the prerequisites list for java.base-jmod.

Fixing this won't impact the build much, but certainly won't hurt either.

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Commit messages:
 - JDK-8310379

Changes: https://git.openjdk.org/jdk/pull/14561/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310379
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14561.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14561/head:pull/14561

PR: https://git.openjdk.org/jdk/pull/14561