[internal/project] DocumentPath in tasks is consistent

[adambabik]

Recently, we discovered some inconsistencies when observing paths returned from runme ls and runme tui, and more precisely from Project.Load() from the internal/project package.

The special case is when the project is bootstrapped by walking the directory upwards. However, the same thing happens if --project is an absolute path.

There are two options:

• Always return absolute paths and do post processing by the caller.
• Always return relative paths; relative to the Project.Root().

cc @sourishkrout

[sourishkrout]

• Always return absolute paths and do post processing by the caller.

let's do this please. thank you @adambabik!