Support import_prefix and strip_import_prefix attrs in jsonnet_library

In light of #139 and #154, I think we should add `import_prefix` and `strip_import_prefix` attrs to `jsonnet_library` to bring virtual includes paths to jsonnet.

`proto_library` and `cc_library` had exact same issues in the past, and prefix magic solved the problem. I believe we can do the same for jsonnet.

Speaking of this, I think we also need to improve `imports` a little bit to make it similar to `cc_library` in some sense.

No implementation file or test is named. Start by comparing the existing jsonnet_library behavior with the referenced proto_library and cc_library prefix handling, then clarify how import_prefix, strip_import_prefix, and imports should interact. Done means the requested virtual include paths work consistently and the behavior is covered by tests.

Written by the indexing model from the issue text.