RFR: 8296390: Incremental build failed with a NPE

[Jan Lahoda]

In [Enter](https://github.com/openjdk/jdk/blob/22347e46f7e66a864ea987fa084c44792cae2e6a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Enter.java#L617), we finish verification that imports in files are valid. But, if the module graph is not finished yet (modules are not initialized), we defer checking imports in module-infos until the modules are setup.

What can happen is that `java.base`', if load lazily, may not be completed as part of module system initialization, and can trigger imports check in other module prematurely.

The proposal is to explicitly complete `java.base` while setting up the compile-time module system. Other (named) modules are normally completed around the same time while [building all modules](https://github.com/openjdk/jdk/blob/22347e46f7e66a864ea987fa084c44792cae2e6a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Modules.java#L1414).

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Commit messages:
 - 8296390: Incremental build failed with a NPE

Changes: https://git.openjdk.org/jdk/pull/10985/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10985&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296390
  Stats: 40 lines in 2 files changed: 39 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10985.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10985/head:pull/10985

PR: https://git.openjdk.org/jdk/pull/10985