RFR: 8306914: Implementation of JEP Launch Multi-File Source-Code Programs

[Christian Stein]

Please review this change set implementing [JEP 458](https://openjdk.org/jeps/458) "Launch Multi-File Source-Code Programs" by introducing a new entry-point `SourceLauncher` in package `com.sun.tools.javac.launcher` of module `jdk.compiler`. The former `Main` entry-point is kept as-is in order to preserve (and possibly fall back to) existing behaviour of launching single-file source-code programs.

The code of the new entry-point and other new classes introduced by this pull request is based on the original implementation. It extends it to dynamically resolve required types "on-the-fly" using an in-memory class loader capable to compile Java source files - applying a strict name-based pattern as described in the JEP.

To support modular programs with user-provided additional modules (launched for example via `java -p . pkg/Prog1.java`) `ALL-MODULE-PATH` is now also part of the implicit list of modules name in source launch mode.

### Open Ends

- [ ] Tests with [JEP ?: Implicitly Declared Classes and Instance Main Method (2nd Preview)](https://bugs.openjdk.org/browse/JDK-8315398)

### OpenJDK's Demo Programs

OpenJDK's demo programs can be found in the [src/demo/share/jfc](https://github.com/openjdk/jdk/tree/master/src/demo/share/jfc) directory. These multi-file source-code programs can be launched via `java ${PROGRAM}` once this implementation is integrated; with `${PROGRAM}` being one of the following files:

- [x] `src/demo/share/jfc/CodePointIM/CodePointIM.java`
- [x] `src/demo/share/jfc/FileChooserDemo/FileChooserDemo.java`
- [x] `src/demo/share/jfc/Font2DTest/Font2DTest.java`
- [x] `src/demo/share/jfc/J2Ddemo/java2d/J2Ddemo.java`
- [x] `src/demo/share/jfc/Metalworks/Metalworks.java`
- [x] `src/demo/share/jfc/Notepad/Notepad.java`
- [x] `src/demo/share/jfc/SampleTree/SampleTree.java`
- [ ] `src/demo/share/jfc/Stylepad/Stylepad.java` — requires `src/demo/share/jfc/Notepad/Notepad.java`
- [x] `src/demo/share/jfc/SwingSet2/SwingSet2.java`
- [ ] `src/demo/share/jfc/TableExample/TableExample.java` — requires a database driver on the class path
- [ ] `src/demo/share/jfc/TableExample/TableExample2.java` —  requires a database driver on the class path
- [x] `src/demo/share/jfc/TableExample/TableExample3.java`
- [x] `src/demo/share/jfc/TableExample/TableExample4.java`
- [x] `src/demo/share/jfc/TransparentRuler/transparentruler/Ruler.java`

-------------

Commit messages:
 - Require package name of the program to match the end of its source file path
 - Revert change to add `ALL-MODULE-PATH`in source launch mode
 - Move test program to directory matching its package name
 - In source launch mode, add all modules on the module path by default
 - 8306914: Launch Multi-File Source-Code Programs

Changes: https://git.openjdk.org/jdk/pull/13712/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13712&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306914
  Stats: 2368 lines in 25 files changed: 2338 ins; 11 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13712.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13712/head:pull/13712

PR: https://git.openjdk.org/jdk/pull/13712