ZX: drop dep

zx drop dep PackageID

Drop a dependency from the current project entirely.

REMEMBER: Because dependencies are statically defined, version updates to existing deps, inclusion of new deps and removal of existing deps require (at least) a patch version increase to the dependent project and submission of the new patch version of the package to the hosting realm.

ceverett@takoyaki:~/code/foo$ zx list deps
otpr-balls-0.3.1
otpr-bar-3.2.0
otpr-baz-1.3.2
ceverett@takoyaki:~/code/foo$ zx drop dep otpr-bar-3.2.0
ceverett@takoyaki:~/code/foo$ zx list deps
otpr-balls-0.3.1
otpr-baz-1.3.2
ceverett@takoyaki:~/code/foo$