[NEW] mail/aerc 0.4.0

With the arrival of the new Go ports tools, I wanted to try mail/aerc
again. Here's my attempt. Notes:

- Because upstream uses git tags like 0.4.0 instead of v0.4.0,
proxy.golang.org relegates him to v0.0.0-longsuffix hell instead of
letting him use real versions. I have no opinion on this important
bikeshed, but the MODGO_VERSION you see here is in fact 0.4.0.

- portgen go based its module list on go.mod from the only version it
was able to fetch, the latest commit, but I changed the modules back
to what they were in 0.4.0.

- The previous time around, with a vendor directory, I was able to patch
the go-libvterm module to link against devel/libvterm instead of
bringing along its own libvterm. I am no longer quite sure how to do
this with the newer Go ports tools because patches apply to WRKDIST,
which doesn't resolve to a directory that actually exists at any point
during the build; if I come up with a fix I'll email it here.