Support all of `pub global activate`'s cli parameters

[seaneagan]

Strawman:

Pub.global.activate('git://github.com/foo/unreleased.git', source: PackageSource.git)
new PubApp.global('git://github.com/foo/unreleased.git', source: PackageSource.git)

enum PackageSource { hosted, git, path }

[devoncarew]

Sounds good!

[seaneagan]

Also would make sense to pass a version constraint ala pub global activate test ^0.12.0. Likely we just add a VersionConstraint constraint optional named parameter to the above APIs as well. You can only pass one or the other of these optional parameters.

[seaneagan]

...where VersionConstraint is from pub_semver.