internal/imports: add a warning not to remove the fixImports seam

CL 589975 accidentally removed an extensibility seam that was used
internally at Google. Strengthen the comment to make it more apparent
that this seam is necessary.

Change-Id: I7bb402a717968eec76cda514cddb4268c67b7329
Reviewed-on: https://go-review.googlesource.com/c/tools/+/590816
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/imports/fix.go

@@ -564,6 +564,8 @@ func (p *pass) addCandidate(imp *ImportInfo, pkg *packageInfo) {
 //
 // This is declared as a variable rather than a function so goimports can
 // easily be extended by adding a file with an init function.
+//
+// DO NOT REMOVE: used internally at Google.
 var fixImports = fixImportsDefault
 
 func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) error {