pass correct (non "main") importpath to nogo for main package

[Strum355]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

In #2135, the importmap passed to emit_compilepkg is "main" if is_main is set (typically via the go_binary rule). This value also gets sent to nogo, which is the wrong value to send for a main package:

Minimal reproduction using x/tools/go/analysis/singlechecker

// go.mod
module sampletext

go 1.21.6

require golang.org/x/tools v0.18.0
require golang.org/x/mod v0.15.0 // indirect

// main.go
package main

import (
	"fmt"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/singlechecker"
)

var Analyzer = &analysis.Analyzer{
	Name: "test", 
    Doc:  "test", 
    Run: func(p *analysis.Pass) (interface{}, error) {
		fmt.Println(p.Pkg.Path())
		return nil, nil
	},
}

func main() { singlechecker.Main(Analyzer) }

$ go run main.go .
sampletext

Which issues(s) does this PR fix?

No issue opened. Should I open one alongside this PR?

Other notes for review

My original note of the issue: sourcegraph/sourcegraph#54836 (comment)
PR where I introduced a first-iteration patch: https://github.com/sourcegraph/sourcegraph/pull/58971/files
I'd been meaning to open a PR for this here for some time, but only found the time to do so now. Apologies!

[Strum355]

Failing tests appear to be due to grailbio/bazel-toolchain being move to bazel-contrib/toolchains_llvm

https://bazelbuild.slack.com/archives/CA31HN1T3/p1707845173596379

[fmeum]

Looks good, I will merge after fixing the pipeline.