RFR: Fix issue with missing JSON classes in make build

[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 introduced a separate call to `find` to find all the sources, so that, if we add another package in the future, we won't have the same problem again.

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

Commit messages:
 - Use find
 - Fix issue with missing JSON classes in make build

Changes: https://git.openjdk.org/jextract/pull/259/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=259&range=00
  Stats: 9 lines in 2 files changed: 4 ins; 4 del; 1 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