RFR: 8258411: Move module set configuration from Modules.gmk to conf dir

[Magnus Ihse Bursie]

The hard-coded list of modules in `make/common/Modules.gmk` has always been a wart in the build system. We pride ourself on using discovery instead of hard-coded list. In this case, it is not possible to do do auto-discovery, since the different module sets are configured, not determined.

Thus the actual lists of module sets should move to the `make/conf` directory.

This is the first patch in a series where I will move configuration values spread over the build system into the designated `make/conf` directory.

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Commit messages:
 - 8258411: Move module set configuration from Modules.gmk to conf dir

Changes: https://git.openjdk.java.net/jdk/pull/1781/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1781&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258411
  Stats: 348 lines in 2 files changed: 179 ins; 162 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1781/head:pull/1781

PR: https://git.openjdk.java.net/jdk/pull/1781