fix: change directory name from aquaproj to aqua

[suzuki-shunsuke]

• Support aggregating configuration files in one directory #1595 (comment)
• feat: support aggregating configuration files in one directory #1615

I reconsidered the directory name.
I think aqua is better than aquaproj, because aqua is simple and easy to understand.
I had a concern the conflict with other projects, but the conflict would rarely occur.

Even if the directory name conflicts, aqua still works.
And you can also separate directory by the prefix ..

aqua # For another project
.aqua # For aqua