fix(add): always remove `destYalcCopyDir` entirely

[aleclarson]

While using #66, I got an error when running yalc add or yalc link with a package that had already been installed with Yalc. This commit fixes that issue.

The relevant commit: 63e81b9e3d4225993329051c44f0bab1f3f1f54d

(Probably wanta merge #59 first)