RFR: Fix issue with missing JSON classes in make build [v2]

[Jorn Vernee]

> Currently, the `make` build is broken, since we added a new package, but the make build doesn't build the source files in that package. 
> 
> This patch adds compilation of the new sources, and adds the missing `--add-exports` flags when running tests. Instead of listing each package in the javac command, I've switched to module source path-based compilation. So, if we ever change the packages in the future, we should run into the same problem again.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  use module-based compilation

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/259/files
  - new: https://git.openjdk.org/jextract/pull/259/files/799584c7..0f93288a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=259&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=259&range=00-01

  Stats: 7 lines in 1 file changed: 1 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/259.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/259/head:pull/259

PR: https://git.openjdk.org/jextract/pull/259