Extend Interface Coverage of LegacyAddressMapper

[JieGhost]

When testing v2 engine in master, I found that LegacyAddressMapper does not have scan_build_files method, which is used by a few tasks. To make all integration tests in master pass on v2 engine, we have to extend the interface coverage of LegacyAddressMapper.

As part of the effort, LegacyAddressMapper should be v2 engine backed (similar to LegacyBuildGraph), which means the new scan_build_files method, along with all existing methods, should be implemented using v2 engine directly.

[JieGhost]

This blocks #3740

[JieGhost]

Merged.