Interface files can depend on unnecessary module imports

[PaulBone]

The build system will cause an interface file to depend on all the module imports included by the module whose interface is being built, rather than only those that interface items depend upon.

There's probably two steps here.

• Make sure that building an interface file doesn't open typeres info it doesn't need (lazy open imports?)
• do_make_dep_info should generate the minimal necessary subset of imports for typeres_import.