Deprecation warnings are emitted for anyone using the old locations.
They'll still work.

The reason for moving the code out of interface is that connectivity
checking is becoming complicated and will have some additions soon. It
feels easier to manage outside of the already fairly complicated
interface handling code.

This also simplifies what happens when the connectivity check fails. All
errors are handled the same and there are no shortcuts to `:lan` even if
it looks like the interface is removed. The expectation is that the
interface will be properly removed shortly if it's for real and
everything else is suspect.

Moving the check handling logic into its own module is intended to make
it easier to adjust it and to simplify addng the Inspector check to the
InternetChecker.

This is a new module that can determine whether a network interface is
connected to the internet by watching traffic on TCP sockets known to
Erlang/OTP. If a device is using long-lived TCP connections (like for
MQTT) that send keepalive messages, then it can declare
internet-connectivity just by watching to the tx and rx counters on the
long-lived connection. This helps reduce use of metered connections and
benefits devices behind restrictive firewalls that drop the pings.

This commit only adds the module. It is hooked up in a subsequent commit.

This majorly reduces internet traffic for checking status on those
network interfaces with long-lived TCP connections. Basically, if a TCP
connection on the interface that's to connecting to a host on a
different LAN has sent and received bytes since the last internet check,
then the check is skipped. Internet is available. AWS IoT and NervesHub
connections are sufficient since their keepalive messages will satisfy
this check.

The connectivity checker is pickier now on whether the interface is up
or down so the unit tests can't take as many shortcuts.

This also fixes an issue where `ifup` would be called and then a
connectivity check done immediately afterwards. The intended behavior is
for the CheckLogic to decide rather than this to be assumed. The result
of this is that the check will be done 500 ms later. This seems good
since it's short and 500 ms allows for a little time for the interface
to settle after it just declared that it was up.