Separate SCIP/Semanticdb symbols on a typelevel in the converter

[keynmol]

While https://github.com/sourcegraph/scip-java/pull/570/files fixes the issue after the fact, we should strive to make the codebase more robust by avoiding using String for everything.

Given that we target Java 8, I don't think we can do much better than a handrolled wrapper class for either, e.g.

public class SemanticdbSymbol {
  private final String raw;

  public SemanticdbSymbol(String raw) {
    this.raw = raw;
  }

  public String getRaw() {
    return raw;
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    SemanticdbSymbol that = (SemanticdbSymbol) o;
    return Objects.equals(raw, that.raw);
  }

  @Override
  public int hashCode() {
    return Objects.hash(raw);
  }
}

The stuff added in #570 will actually serve as an excellent safety net for aiding with this refactoring