Unify improvers

[TG1999]

          Why not have these classes as seperate files in the `improvers` directory? I only see the `oval` improver has it's own file, is that only because of it's size? We would probably want to be consistent here and have the improvers in their respective directory unless that's inconvenient/not possible because of something.

Originally posted by @AyanSinhaMahapatra in #1102 (comment)

[uzaxirr]

Hey can i work on this?

from what i understood is that you want to club all the classes which inherits form vulnerabilities.improver.Improver class to the improvers directory with a separate file for each. Much like vulnerabilities/improvers/oval.py
and also modify the import statements where required.

Please correct me if i'm wrong.

[TG1999]

@uzaxirr we want a single improver that could help us to resolve the version ranges for the ecosystems that we store for example there's some repeated code in every improver which could be clubbed into a single improver with some modifications.

[TG1999]

Closing this, it's done now.