[tfa] After TFA a `library.dependencies` may contain many dependencies that are unused

[mkustermann]

Currently TFA's tree shaker seems to do do something like this: If a library A used to import library B but B was tree shaken, then it will inject into A the library imports from transitive closure of B that were not tree shaken. By doing that it maintains something like this not-tree-shaken-libraries(transitive(A(before-TFA))) == transitive(A(after-TFA)).

Now the issue is that this makes a library have dependencies/imports that are unused by the library.

Some backends (e.g. VM) use that resulting Library.dependencies for computing deferred loading units. So by having these extra dependencies it may produce suboptimal deferred loading units splitting.

See also more broad discussion #62112

/cc @alexmarkov