bndmanifest and java-library intermodule dependencies

In https://github.com/diffplug/durian-swt/pull/7/commits/496386ecebd023075b03a041d238af8d02a29469 I used `java-library` in a multi-module project, and I declared an `api` dependency on another module within the project. I then saw intermittent errors like [this](https://travis-ci.org/diffplug/durian-swt/builds/636211276#L237):

```
> Task :durian-swt:jar FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':durian-swt:jar'.
> A Jar can only accept a file or directory that exists: /home/travis/build/diffplug/durian-swt/durian-swt.os/build/libs/durian-swt.os-3.3.0.jar
```

The workaround is https://github.com/diffplug/durian-swt/pull/7/commits/b3df3311d6ef3d6ce22756512a7e742227091e3c