feat(impact): Go callers from syntax facts

A Go package is its directory, and an import binds it by the path go.mod
gives it, an alias or a dot import. An unexported name never leaves its
package. A method call resolves when the syntax states the value's type
(a receiver, a parameter, `var x T`, `x := &pkg.T{}`), and is a
candidate from any file that can see the method's package otherwise. A
same-named function of another package, a struct field, a local and a
string are no longer callers.

Gate: go_callers_bind_through_the_package_and_the_stated_type, with
decoys of each kind; the lexical build finds 0 of 3 Total callers and
lists the field decoy, and finds no Reconcile caller.

Refs #506

Author: nikolay-e

CHANGELOG.md

@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Go callers come from syntax facts: a package is its directory, an import
+  binds it by the path `go.mod` gives it, an alias or a dot import names it,
+  an unexported name never leaves its package, and a method call resolves
+  when the syntax states the value's type (a receiver, a parameter,
+  `x := &pkg.T{}`). A same-named function of another package, a struct
+  field, a local and a string are no longer callers (#506).
+
 ### Fixed
 
 - Module-level calls are listed beside a function-level caller in the same