Update MacPorts base using HTTP

[neverpanic (Clemens Lang)]

In #60590, we again noticed that our current method to update MacPorts base source code via rsync is not optimal, since we do not have automatic fallbacks to other mirrors should our main mirror be unavailable.

Additionally, the use of rsync is (a) not transport encrypted, and (b) often requires special care for the proxy configuration, while the proxy configuration for HTTP is usually already set.

Syncing the entire source code via rsync and only then checking the current version in a file in the downloaded source tree is also rather slow, compared to directly checking the current version from some online source.

We should switch to using pure HTTP to check for the current version, and then download a tarball of the new version once it is available. Obviously, the signature checking that is currently in place will need to be kept.

[neverpanic (Clemens Lang)]

​https://github.com/macports/macports-base/pull/184

[ryandesign (Ryan Schmidt)]

Thank you for suggesting this and especially for taking the time to write the code. I haven't had time to look at the code yet.