Fix CSS Fragment Identifier False Warnings

Problem

When using CSS libraries like Leaflet, users see spurious "Unable to resolve module" warnings:

Unable to resolve some modules:
  "#default#VML" in /packages/...leaflet.css (web.browser)

This occurs with CSS containing fragment identifiers in url() values, particularly IE-specific behaviors like behavior: url(#default#VML);

Root Cause

• Meteor's import scanner treats CSS fragment identifiers (starting with #) as module imports
• Fragment identifiers are valid CSS but never represent JavaScript modules

Solution

Added check in PackageSourceBatch._warnAboutMissingModules() to skip IDs starting with #.

Changes Made

• Added early return for identifiers starting with #
• No impact on actual module resolution, only suppresses false warnings

Fixes #14037