PRESERVE_TOPLEVEL broke adding requirements #17
Description
Commit 0b86ba7 apparently introduced a bug with the add command, where if no other flags are provided, the provided specifier replaces all of the requirements in the targeted source file.
I have a very fuzzy recollection of why that configuration flag was introduced, and only exposed via the environment, and not documented. It certainly is not working as intended, so for now I'm just removing it altogether.
There is also a bug in requirements.resolve_specifier, in which it does not return a HashableInstallRequirement sometimes, resulting in the possibility of duplicate requirements being added.