Write tests to demonstrate how baseUrl + pathMapping to node_modules behaves

Author: sheetalkamat

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.js

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+//// [tests/cases/compiler/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.ts] ////
+
+//// [foobar.js]
+module.exports = { a: 10 };
+
+//// [a.ts]
+import foobar from "foo/bar/foobar.js";
+
+
+//// [/bin/node_modules/foo/bar/foobar.js]
+module.exports = { a: 10 };
+//// [/bin/a.js]
+"use strict";
+exports.__esModule = true;

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.symbols

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+=== /a.ts ===
+import foobar from "foo/bar/foobar.js";
+>foobar : Symbol(foobar, Decl(a.ts, 0, 6))
+
+=== /node_modules/foo/bar/foobar.js ===
+module.exports = { a: 10 };
+>module.exports : Symbol("/node_modules/foo/bar/foobar", Decl(foobar.js, 0, 0))
+>module : Symbol(export=, Decl(foobar.js, 0, 0))
+>exports : Symbol(export=, Decl(foobar.js, 0, 0))
+>a : Symbol(a, Decl(foobar.js, 0, 18))
+

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.trace.json

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+[
+    "======== Resolving module 'foo/bar/foobar.js' from '/a.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "'baseUrl' option is set to '/', using this value to resolve non-relative module name 'foo/bar/foobar.js'.",
+    "'paths' option is specified, looking for a pattern to match module name 'foo/bar/foobar.js'.",
+    "Module name 'foo/bar/foobar.js', matched pattern '*'.",
+    "Trying substitution 'node_modules/*', candidate module location: 'node_modules/foo/bar/foobar.js'.",
+    "File '/node_modules/foo/bar/foobar.js' exist - use it as a name resolution result.",
+    "======== Module name 'foo/bar/foobar.js' was successfully resolved to '/node_modules/foo/bar/foobar.js'. ========"
+]

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.types

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+=== /a.ts ===
+import foobar from "foo/bar/foobar.js";
+>foobar : { a: number; }
+
+=== /node_modules/foo/bar/foobar.js ===
+module.exports = { a: 10 };
+>module.exports = { a: 10 } : { a: number; }
+>module.exports : { a: number; }
+>module : { "/node_modules/foo/bar/foobar": { a: number; }; }
+>exports : { a: number; }
+>{ a: 10 } : { a: number; }
+>a : number
+>10 : 10
+

tests/cases/compiler/pathMappingBasedModuleResolution_withExtension_MapedToNodeModules.ts

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+// @noImplicitReferences: true
+// @traceResolution: true
+// @allowJs: true
+// @esModuleInterop: true
+// @fullEmitPaths: true
+
+// @Filename: /node_modules/foo/bar/foobar.js
+module.exports = { a: 10 };
+
+// @Filename: /a.ts
+import foobar from "foo/bar/foobar.js";
+
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "baseUrl": ".",
+        "paths": {
+            "*": ["node_modules/*", "src/types"]
+        },
+        "allowJs": true,
+        "outDir": "bin"
+    }
+}