feat(javac): Add java_javac_sourcepath variable

[nibocn]

This variable can set multiple source code paths, the source code path is a relative path (relative to the project root directory)

[hsanson]

The classpath variable works with a string instead of an array. For consistency I think would be good to make this new but related variable too a string or even better make both variables classpath and sourcepath support both strings and arrays.

[nibocn]

The classpath variable works with a string instead of an array. For consistency I think would be good to make this new but related variable too a string or even better make both variables classpath and sourcepath support both strings and arrays.

Okay, I will adjust the sourcepath variable to string later

[hsanson]

Good improvement on javac configuration.

[hsanson]

@nibocn Are you planning more changes? if not I can merge this change.

[nibocn]

@hsanson That's all, thanks.