build: fix error when running devapp on windows (#6288)

Currently developers using Windows are not able to serve the devapp because the secondary entry-points can't be resolved. This can happen because of two things:

* `egrep`, which is an alias for `grep -E` is not part of the global `$PATH` variable on Windows
* The Regular Expression for the `grep` call is ambiguous and expects zero-width characters to be present.

This commit changes the `spawnSync` call to use `grep -E` and also changes the Regular Expression to be more explicit about what characters to match after the `from`.

tools/package-tools/secondary-entry-points.ts

@@ -38,10 +38,10 @@ export function getSecondaryEntryPointsForPackage(pkg: BuildPackage) {
   buildNodes.forEach(node => {
     // Look for any imports that reference this same umbrella package and get the corresponding
     // BuildNode for each by looking at the import statements with grep.
-    node.deps = spawnSync('egrep', [
-      '-roh',
+    node.deps = spawnSync('grep', [
+      '-Eroh',
       '--include', '*.ts',
-      `from.'@angular/${packageName}/.+';`,
+      `from '@angular/${packageName}/.+';`,
       `${packageDir}/${node.name}/`
     ])
     .stdout