RFR: 8177650: JShell tool: packages in classpath don't appear in completions

[Jan Lahoda]

JShell provides the code completion feature, where it suggests possible follow ups for a given snippet prefix. To allow completion for packages, JShell uses a background task to go through known classes and create an index for them.

There are two problems with this background task:
- the classpath is read from the JShell's FileManager, but the FileManager may not be configured with the compile options yet, so the classpath may not be filled yet,
- the module path is not included in the list of paths to paths to search

This PR proposes to:
- use FileManager configured by passing the compile options through javac
- include the module path in the search path

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Commit messages:
 - Correcting handling of modular paths.
 - Improving test reliability.
 - 8177650: JShell tool: packages in classpath don't appear in completions

Changes: https://git.openjdk.org/jdk/pull/26137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8177650
  Stats: 281 lines in 5 files changed: 278 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26137/head:pull/26137

PR: https://git.openjdk.org/jdk/pull/26137