cmd/go: build panics when passed `="` as build path

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

Does this issue reproduce with the latest release?

yes (1.17.2)

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

What did you do?

run either of these commands in any go project dir

go build '' -ldflags='-X "main.Tags="'
go install '' -ldflags='-X "main.Tags="'

What did you expect to see?

The compilation should succeed, as it does when removing the ''.

-go build '' -ldflags='-X "main.Tags="'
+go build -ldflags='-X "main.Tags="'

What did you see instead?

panic: path "-ldflags=-X \"main.Tags=\"" not in error "invalid import path \"-ldflags=-X \\\"main.Tags=\\\"\"" [recovered]
	panic: path "-ldflags=-X \"main.Tags=\"" not in error "invalid import path \"-ldflags=-X \\\"main.Tags=\\\"\""

goroutine 1 [running]:
cmd/go/internal/load.(*preload).flush(0xc0001f62c0)
	/usr/lib/go/src/cmd/go/internal/load/pkg.go:1041 +0x78
panic({0x8f9d60, 0xc000684a60})
	/usr/lib/go/src/runtime/panic.go:1038 +0x215
cmd/go/internal/load.ImportErrorf({0x7ffca7ec715d, 0x18}, {0x99f456, 0x18}, {0xc00021f6b0, 0xc00021fb58, 0x0})
	/usr/lib/go/src/cmd/go/internal/load/pkg.go:500 +0x19b
cmd/go/internal/load.(*Package).load(0xc0006dc000, {0xa70560, 0xc000122000}, {0x34, 0xc0, 0x2}, {0x7ffca7ec715d, 0x0}, 0xc00021fb58, {0x0, ...}, ...)
	/usr/lib/go/src/cmd/go/internal/load/pkg.go:1846 +0x17dc
cmd/go/internal/load.loadImport({0xa70560, 0xc000122000}, {0x60, 0x5, 0xa7}, 0xc0001f62c0, {0x7ffca7ec715d, 0x18}, {0xc00002c034, 0x24}, ...)
	/usr/lib/go/src/cmd/go/internal/load/pkg.go:718 +0x4dd
cmd/go/internal/load.PackagesAndErrors({0xa70560, 0xc000122000}, {0x2d, 0x0, 0x0}, {0xc000134030, 0x2, 0x2})
	/usr/lib/go/src/cmd/go/internal/load/pkg.go:2468 +0x809
cmd/go/internal/work.runBuild({0xa70560, 0xc000122000}, 0xc000150420, {0xc000134030, 0x2, 0x2})
	/usr/lib/go/src/cmd/go/internal/work/build.go:371 +0xa5
main.invoke(0xd804e0, {0xc000134010, 0x4, 0x4})
	/usr/lib/go/src/cmd/go/main.go:216 +0x2f6
main.main()
	/usr/lib/go/src/cmd/go/main.go:173 +0x78e