Some input filesystems don't implement purge

[halfnelson]

If the input filesystem doesn't have purge, the current code throws.

C:\dev\nativescript\svelte-ns-testapp\node_modules\svelte-loader\lib\virtual.js:15
                        originalPurge.call(this, arguments);
                                      ^

TypeError: Cannot read property 'call' of undefined
    at Object.VirtualModulesPlugin.compiler.inputFileSystem.purge (C:\dev\nativescript\svelte-ns-testapp\node_modules\svelte-loader\lib\virtual.js:15:18)
    at Compiler.purgeInputFileSystem (C:\dev\nativescript\svelte-ns-testapp\node_modules\webpack\lib\Compiler.js:302:25)
    at compilerCallback (C:\dev\nativescript\svelte-ns-testapp\node_modules\webpack-cli\bin\cli.js:483:15)

I encountered this problem. This commit solved the problem

[benbot]

And word as to why this hasn't been merged in yet?

Could it potentially break something?