Support more remote types in use_dev_package.

[antoine-sachet]

Any remote of the form 'type::username/repo' is now supported by use_dev_package. Close #970.

The internal function package_remote only checks that the RemoteType, RemoteUsername and RemoteRepo are non-NA scalar string.

This means:

• All standard remotes like bitbucket, gitlab, etc will work just fine.
• Github remotes will now be prefixed by github:: in the DESCRIPTION
• As requested, github remotes will not get the github:: prefix, in line with the existing behaviour.

[hadley]

Thanks @antoine-sachet!