fix(ngcc): do not collect private declarations from external packages (#34811)

Previously, while trying to build an `NgccReflectionHost`'s
`privateDtsDeclarationMap`, `computePrivateDtsDeclarationMap()` would
try to collect exported declarations from all source files of the
program (i.e. without checking whether they were within the target
package, as happens for declarations in `.d.ts` files).

Most of the time, that would not be a problem, because external packages
would be represented as `.d.ts` files in the program. But when an
external package had no typings, the JS files would be used instead. As
a result, the `ReflectionHost` would try to (unnecessarilly) parse the
file in order to extract exported declarations, which in turn would be
harmless in most cases.

There are certain cases, though, where the `ReflectionHost` would throw
an error, because it cannot parse the external package's JS file. This
could happen, for example, in `UmdReflectionHost`, which expects the
file to contain exactly one statement. See #34544 for more details on a
real-world failure.

This commit fixes the issue by ensuring that
`computePrivateDtsDeclarationMap()` will only collect exported
declarations from files within the target package.

Jira issue: [FW-1794](https://angular-team.atlassian.net/browse/FW-1794)

Fixes #34544

PR Close #34811

packages/compiler-cli/ngcc/test/host/commonjs_host_spec.ts

@@ -2184,7 +2184,7 @@ exports.ExternalModule = ExternalModule;
         });
       });
 
-      describe('getDtsDeclarationsOfClass()', () => {
+      describe('getDtsDeclaration()', () => {
         it('should find the dts declaration that has the same relative path to the source file',
            () => {
              loadTestFiles(TYPINGS_SRC_FILES);