Define src/ as source code root

Summary:
This aligns the repository with conventions used by other Keruald repositories.

Ref T2035.

Test Plan:

• Call XHubSignature from psysh, class is found
• Run tests

Reviewers: aceppaluni

Reviewed By: aceppaluni

Maniphest Tasks: T2035

Differential Revision: https://devcentral.nasqueron.org/D3527