Move code out of the `postfix/` subdirectory

[rhansen]

This makes it easier to generate patches that apply with the usual patch -p1, and to cross-develop with (cherry-pick to/from) repositories such as the Debian packaging repository at https://salsa.debian.org/postfix-team/postfix-dev.

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Alternative to this PR:

• Create a new upstream branch that has the non-prefixed and unmodified contents of each release tarball (no .gitignore).
• Whenever upstream is updated, perform a subtree merge into master:

  git checkout master && git merge -Xsubtree=postfix/ upstream