Fix handling of java files without package name.

Instead of returning null, we return the filename
as pacakgePrependedFileName for such files.

Also, inludes a fix for package name regex.

Bug: 426532567
Test: atest dependency-mapper-tests
Flag: EXEMPT bugfix
Change-Id: I21ba25dbb55cd8c53b5d6f44cbeda9abbad3306a