RFR: 8303588: [JVMCI] make JVMCI source directories conform with standard layout

[Doug Simon]

The layout of the sources in `jdk.internal.vm.ci` stems from their initial development outside the JDK where they adopted a layout influenced by Eclipse.

There's no good reason for maintaining this layout any more. Moving to a standard layout also means IDEs will be able to make sense of the JVMCI sources in `src.zip`.

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Commit messages:
 - made JVMCI source directories conform with standard layout

Changes: https://git.openjdk.org/jdk/pull/12860/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12860&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303588
  Stats: 20 lines in 209 files changed: 0 ins; 20 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12860.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12860/head:pull/12860

PR: https://git.openjdk.org/jdk/pull/12860