feat: add support for fully qualified urls for Mint packages

[kronenthaler]

Changes

Mintfiles support fully qualified urls to specify swift dependencies (some might be in bitbucket or in on-premise hosted git repos)
The current mint manager was constructing blindly a url prefixing https://github.com/ creating weird and incorrect urls e.g.
https://github.com/https://bitbucket.org/atlassian/swift-http-server.git.git

This PR allows the manager to detect fully qualified urls and pass them as they are defined.

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[viceice]

this can cause false positive if a normal dep starts with http