dependency overrides

[cjdoris]

Allow something like

[deps.somepackage]
version = ">=1.2, <2"
override = true

in CondaPkg.toml. The flag override=true has no effect unless it's in the top-level environment, in which case this dependency overrides any other dependencies for the same package name. And similar for pip deps.

Also some mechanism to avoid installing a package? Maybe a special version = "skip"?

[cjdoris]

slightly related to #114 - in that there are various overrides going in CondaPkg.toml or Preferences.toml files