feat(nix/buildGo): add the import position to errmsg

When a foreign dep is missing a dependency, it is good to have a
context.

e.g. the `github.com/charmbracelet/bubblegum` package has a lot of
dependencies that are only used in its `examples/` dir; this is not
obvious, unless we also print where the imports come from.

New error message:

```
error: missing foreign dependency 'github.com/containerd/console' in 'github.com/charmbracelet/bubbletea, imported at /nix/store/0cry4sg6bzxqwk5zl2nxhas6k5663svg-source/tea.go:22:2'
```

Change-Id: If34a3c62b9d77d4aea108b5e011e16fbd03e8554
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2852
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/buildGo/external/default.nix

@@ -56,8 +56,8 @@ let
         throw "missing local dependency '${lib.concatStringsSep "." d}' in '${path}'"
       ) self) entry.localDeps;
 
-      foreignDeps = map (d: lib.attrByPath [ d ] (
+      foreignDeps = map (d: lib.attrByPath [ d.path ] (
-        throw "missing foreign dependency '${d}' in '${path}'"
+        throw "missing foreign dependency '${d.path}' in '${path}, imported at ${d.position}'"
       ) depMap) entry.foreignDeps;
 
       args = {

nix/buildGo/external/main.go

@@ -34,10 +34,16 @@ type pkg struct {
 	Files       []string     `json:"files"`
 	SFiles      []string     `json:"sfiles"`
 	LocalDeps   [][]string   `json:"localDeps"`
-	ForeignDeps []string   `json:"foreignDeps"`
+	ForeignDeps []foreignDep `json:"foreignDeps"`
 	IsCommand   bool         `json:"isCommand"`
 }
 
+type foreignDep struct {
+	Path string `json:"path"`
+	// filename, column and line number of the import, if known
+	Position string `json:"position"`
+}
+
 // findGoDirs returns a filepath.WalkFunc that identifies all
 // directories that contain Go source code in a certain tree.
 func findGoDirs(at string) ([]string, error) {
@@ -88,7 +94,7 @@ func analysePackage(root, source, importpath string, stdlib map[string]bool) (pk
 	}
 
 	local := [][]string{}
-	foreign := []string{}
+	foreign := []foreignDep{}
 
 	for _, i := range p.Imports {
 		if stdlib[i] {
@@ -100,7 +106,12 @@ func analysePackage(root, source, importpath string, stdlib map[string]bool) (pk
 		} else if strings.HasPrefix(i, importpath+"/") {
 			local = append(local, strings.Split(strings.TrimPrefix(i, importpath+"/"), "/"))
 		} else {
-			foreign = append(foreign, i)
+			// The import positions is a map keyed on the import name.
+			// The value is a list, presumably because an import can appear
+			// multiple times in a package. Let’s just take the first one,
+			// should be enough for a good error message.
+			firstPos := p.ImportPos[i][0].String()
+			foreign = append(foreign, foreignDep{Path: i, Position: firstPos})
 		}
 	}