make vsp library public

[oshorefueled]

Currently the vsp library is in the internal directory making it impossible to use the code outside dcrwallet. Making it public would eliminate the need to replicate it in dclibwallet used by dcrandroid, dcrios and godcr.

[xaur]

I guess this will be addressed by #2237

[xaur]

I believe the plan has changed. decred/vspd#382 introduced a circular dependency that was problematic. As a result decred/vspd#395 and #2237 got canceled, and VSP code that made it into the vspd reporisoty was removed in decred/vspd#403 . I think the new plan is to keep the VSP client in dcrwallet repo and make it an exported/reusable package.

[JoeGruffins]

closed by #2359