Remove all NIO umbrella imports

[fabianfett]

We should not depend on the NIO umbrella module.

Instead we should explicitly

• import NIOCore
• import NIOPosix
• import NIOConcurrencyHelpers
• import NIOEmbedded
• ...

where needed.

We should also add explicit dependencies in the Package.swift

Acceptance criteria:

This code in scripts/soundness.sh can be commented in again and CI passes:

# This checks for the umbrella NIO module.
printf "=> Checking for imports of umbrella NIO module... "
if git grep --color=never -i "^[ \t]*import \+NIO[ \t]*$" > /dev/null; then
    printf "\033[0;31mUmbrella imports found.\033[0m\n"
    git grep -i "^[ \t]*import \+NIO[ \t]*$"
    exit 1
fi
printf "\033[0;32mokay.\033[0m\n"