create release tarballs with sources of all dependencies

Currently when building the package in pmaports, sources of dependencies are downloaded at build time.

Let's add a convenient way to download the sources of dependencies and create proper release tarballs, which we can use in pmaports to avoid this. And document the process how to do it.

https://wiki.alpinelinux.org/wiki/APKBUILD_examples:Go -> let's go for option 3 there, vendor dir with all depends

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