Refactored and added Export-Package header parser

* Renamed parsers to more clearly indicate what they parse.

* Moved test cases to the appropriate package.

* Implemented Export-Package header parser and added unit tests
for it.

* Moved common functionality for parsing imports and exports to
a separate object.

* Used new export parser in ScalaFileImporter.

* Added null checks to avoid NPEs in parsers.

* Fixed typo in FileImporter javadoc comments.
I will add a "USES" relationship between "Package"s.