Feature: Add `debug` support.

[ryan-roemer]

Some ideas:

• Lazy require debug without it being a dependency.
• Use node12+ debug logger in util
• Things to log
  • require(A_VAR)
  • require.resolve(A_VAR)
  • import(A_VAR)
  • ignores matches
  • allowMissing matches

Possible starting point diff:

diff --git a/lib/debug.js b/lib/debug.js
new file mode 100644
index 0000000..b9d36e2
--- /dev/null
+++ b/lib/debug.js
@@ -0,0 +1,15 @@
+"use strict";
+
+/**
+ * Simple debugger using lazy http://npm.im/debug
+ */
+const NOOP = () => {};
+
+let debug;
+try {
+  debug = require("debug")
+} catch (err) {
+  debug = () => NOOP;
+}
+
+module.exports = debug;
diff --git a/lib/extract.js b/lib/extract.js
index 0a26c6b..0bc6c7b 100644
--- a/lib/extract.js
+++ b/lib/extract.js
@@ -3,8 +3,11 @@
 /**
  * Extract dependencies from code.
  */
-
 const walk = require("acorn-node/walk");
+const debug = require("debug");
+
+// Track non-literal imports
+const debugNonLiteral = debug("trace-deps:non-literal");
 
 // Extract all dependency strings from require/import statements.
 const getDeps = (ast) => {
@@ -183,10 +186,12 @@ const getDeps = (ast) => {
     //   value: four
     //   raw: '"four"'
     // ```
-    ExportNamedDeclaration({ source } = {}) {
-      source = source || {};
+    ExportNamedDeclaration(node = {}) {
+      const source = node.source || {};
       if (source.type === "Literal") {
         deps.add(source.value);
+      } else {
+        debugNonLiteral(JSON.stringify(node));
       }
     },
 
@@ -207,6 +212,8 @@ const getDeps = (ast) => {
       source = source || {};
       if (source.type === "Literal") {
         deps.add(source.value);
+      } else {
+        debugNonLiteral(JSON.stringify(source));
       }
     }
 
diff --git a/package.json b/package.json
index be11426..93bf173 100644
--- a/package.json
+++ b/package.json
@@ -33,6 +33,7 @@
     "chai": "^4.2.0",
     "chai-as-promised": "^7.1.1",
     "codecov": "^3.6.5",
+    "debug": "^4.1.1",
     "eslint": "^6.8.0",
     "eslint-config-formidable": "^4.0.0",
     "eslint-plugin-filenames": "^1.3.2",