All,

I'm writing to propose an adjacent interpreter flag to `-X importtime`: `-X importcache` (open to alternative naming). 

While `-X importtime` is incredibly useful for analyzing module import times, by design, it doesn't log anything if an imported module has already been loaded. `-X importcache` would provide additional output for every module that's already been loaded: 

```
>>> import uuid
import time: cached    | cached     |   _io
import time: cached    | cached     |   _io
import time: cached    | cached     |   os
import time: cached    | cached     |   sys
import time: cached    | cached     |   enum
import time: cached    | cached     |     _io
import time: cached    | cached     |     _io
import time: cached    | cached     |     collections
import time: cached    | cached     |     os
import time: cached    | cached     |     re
import time: cached    | cached     |     sys
import time: cached    | cached     |     functools
import time: cached    | cached     |     itertools
import time:       151 |        151 |     _wmi
import time:     18290 |      18440 |   platform
import time:       372 |        372 |   _uuid
import time:     10955 |      29766 | uuid
```

In codebases with convoluted/poorly managed import graphs (and consequently, workloads that suffer from long import times), the ability to record all paths to an expensive dependency--not just the first-imported--can help expedite refactoring (and help scale identification of this type of issue). More generally, this flag would provide a more efficient path to tracking runtime dependencies.

As a proof of concept, I was able to hack this functionality into `-X importtime` by adding a couple lines to `import_ensure_initialized` in `Python/import.c` (hence the output above). A separate flag is probably desirable to preserve backwards compatibility. 

Looking forward to your feedback,
Noah