Generated File "part of" import fix

[cmenkemeller]

This is a bit of an edge case but the resolution should be good for overall generated file consistency.

Issue Replication

Run builder using the following configuration-

1. output_file_name : localizations.t.dart (note: this is purposefully not set to a .g type file)
2. output_format : multiple_files

Resulting files will contain error in "part of" section of imports due to a mismatch in file name. Generated file name fails to pull from the custom output_file_name parameter.

Resolution

Update "part of" code generation to use the config output_file_name instead of manually creating file name using baseName

Updates

1. Added outputFileName as a parameter in GenerateConfig class
2. Replaced "part of" import with outputFileName
3. Updated tests to verify

[Tienisto]

Thank you!