cmd/go: go install -buildmode=plugin creates shared object with wrong file name

[zephyrtronium]

What version of Go are you using (go version)?

$ go version
go version go1.12.7 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/zephyrtronium/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/zephyrtronium/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build282654314=/tmp/go-build -gno-record-gcc-switches"

What did you do?

~/mygo/plugintest$ printf 'package main\n\nvar String = "Hello, world!"\n' >plugin.go
~/mygo/plugintest$ go install -buildmode=plugin -x

What did you expect to see?

A shared object named plugintest.so in /home/zephyrtronium/go/pkg/linux_amd64_dynlink/github.com/zephyrtronium/.

What did you see instead?

A shared object named plugintest.a in the same directory.

I confirmed that the plugin loads correctly using the plugin package, it's just named .a instead of .so. The output from go install -buildmode=plugin -x includes mv $WORK/b001/exe/a.out.so /home/zephyrtronium/go/pkg/linux_amd64_dynlink/github.com/zephyrtronium/plugintest.a, versus go build -buildmode=plugin -n having mv $WORK/b001/exe/a.out.so plugintest.so.

[bcmills]

CC @jayconrod @ianlancetaylor @crawshaw