build: expose contract deployers via curve_stablecoin package

[AlbertoCentonze]

Expose deployers in the built package so that this repo can be used in python scripting:

• Expose all deployers by doing from curve_stablecoin import <DEPLOYER_NAME>
• Refactor the way paths are handled in deployers.py to use the path lib and be more generic.
• As an implementation side effect, the whole test suite is now part of the build, could be useful in some cases.