Require erc-networks in erc.el

Require erc-networks in erc.el

* lisp/erc/erc.el: Require erc-networks.el, which ERC can't run
without these days.  To sidestep the circular dependency, require it
last, just after erc-goodies.  Remove the `declare-function' for
`erc-network-name' because it's not currently needed at load time.
(erc-log-irc-protocol, erc-hide-current-message-p): Remove `fboundp'
guard logic from `erc-network-name' invocations but preserve meaning
by interpreting `erc-network' being unset to mean module isn't loaded
or authoritative network detection has failed.
(erc-format-network): Likewise here.  At the moment, this function
always returns the empty string because the function
`erc-network-name' always returns non-nil, perhaps from the
fallback/failure sentinel "Unknown", perhaps from the printed form of
nil.

* lisp/erc/erc-networks.el (erc-network): This is called throughout
erc.el but was previously cumbersome to use on account of being
guarded by `fboundp'.  It now relies on the fact that its namesake
variable is set in target buffers as well.