FR: add PyInfo.direct_imports

Currently, the PyInfo's imports field returns all imports paths defined in a target, combined with the ones from its transitive dependencies, as defined here

There's no way to retrieve the direct import paths. It would be useful for IDE support, where we typically read data about a large number of targets and dependencies between them. In that case, we pessimistically would end up in n^2 import entries when reading that, because of depset expansion.

Alternatively IDEs could read just direct_imports and deps fields, and map them to their internal graph. Full transitive imports could still be resolved on demand, based on deps

What underlying problem are you trying to solve with this feature?

Avoid depset flattening when reading both imports and deps from PyInfo, according to https://bazel.build/rules/performance#avoid-depset-to-list

(originally from https://github.com/bazelbuild/bazel/issues/20449 by @tpasternak )