deambiguate imported functions

[moodymudskipper]

Use case:

• A project uses a lot of library calls, without conflicted.
• We converted them to @ import
• devtools::check complains that we are overriding a ton of functions

Solution:

• A function to convert all import tags to importFrom
• A function to resolve the ambiguities, by just enumerating them, and we have to remove the redundant entry and check the code manually for usage (maybe we can use this feature similar to find in files, forgot the name, see my aborted PR in golem) so we can add :: where needed

[moodymudskipper]

the keyword is source markers: see https://github.com/ThinkR-open/golem/pull/595/commits/398b5bb2a3a28cd81f6c60df3397ce3571685637