Improve Gson development experience

[Marcono1234]

📢 Feel free to comment your experience (good or bad) below, and share tips and tricks.

---

Gson's build setup has some special configurations:

• src/main/java contains module-info.java, but it is excluded for compilation and processed by Moditect
  (but it is intentionally not excluded for the Javadoc plugin)
  This is done because Gson currently still targets Java 8.
• main sources are compiled with a different target Java version than test sources
• OSGiManifestIT relies on created Gson JAR
• Formatting with Spotless
• ... maybe more

It seems IDEs (Eclipse, IntelliJ, ...) do not handle all of these special configurations well. For example:

• Eclipse always encounters internal exceptions during Maven builds for me
• IntelliJ encounters internal javac assertion errors for test execution, apparently due to the special module-info.java handling

(maybe something is broken with my setup)

Most likely new contributors experience similar issues, and for them it will be more difficult (and discouraging) because they are not familiar with the project yet.

So it would probably be good to investigate and get feedback which issues users encounter while setting up the Gson development environment, and then check if we can improve the project setup (ideally without losing functionality), e.g.:

• adjusting the build setup
• improving / adding documentation