Reorganize `src` directory to add subfolders for each module type

[clemlak]

Description

Add new subfolders for each module type (LiquidityMigrator, PoolInitializer, ...) to the src directory in order to improve the readability of our repository.

Motivation

More and more contracts are being added to the src directory, however only a few subfolders exist and they are currently limited to some specific aspects base, interfaces, etc... This results in the src folder having lots and lots of files and being more and more confusing to work with.

Proposed Solution

src folder could be reorganized to look like this:

src
 ├─base
 ├─interfaces
 ├─lens
 ├─libraries
 ├─modules
 │  ├─governance
 │  ├─initializer
 │  ├─migrator
 │  └─token
 └─types

Note that the extensions folder is no longer present, as it contains modules created by 3rd parties, its content should be removed from the main repository.