Migrate to src layout

[YuevUwU]

As projects grow more complex, we will need better maintainability.
Therefore, it would be a nice try to consider modifying the project layout.

( The plan was first mentioned by @winstonsung )

@YuevUwU wrote:
Any plan about migrating code to src folder, or develop in original path?

@winstonsung wrote:
The current plan is to migrate code to the src folder.
There were attempts about using the flat layout, but it turns out to be more complicated than the src layout, thus being reverted.

To-do

• Discuss whether and when to proceed
• Modify codes to conform the src layout
• Perform thorough testing to confirm that the migration does not break existing functionality
• Updated guide/docs to reflect the new structure

See also

• Packaging: Prepare src layout migration #91