RFR: 8259942: Enable customizations in CompileJavaModules.gmk and Main.gmk

[Adam Farley]

Ensure make files look on the include path or in PHASE_MAKEDIRS for
customizations.

Change also adds a tidy-up that improves readability.

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Commit messages:
 - 8259942: Enable customizations in CompileJavaModules.gmk and Main.gmk

Changes: https://git.openjdk.java.net/jdk/pull/2134/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2134&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259942
  Stats: 5 lines in 3 files changed: 1 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2134.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2134/head:pull/2134

PR: https://git.openjdk.java.net/jdk/pull/2134