Fix some issues in bp2build converter for python_binary_host.

1) Bp2build convert python_binary_host main attribute as LabelAttribute. Currently "main" attribute in python_binary_host is handled as string but for some modules (e.g certify_bootimg) the "main" attribute points to a file in its subpackage like "subpackage/file.py" and should be converted to "//.../subpackage:file.py".

2) Filter out duplicated labels in the merged list of "required" attributes of python_binary_host and its defaults.

Test: b build //system/tools/mkbootimg:certify_bootimg
Test: b build //build/make/tools/releasetools:check_target_files_signatures
Bug: 253081249
Bug: 253101186

Change-Id: Ic2cb4cadec2c1348da70af9f0730da9914d3a8ca