gstplugin: Don't stat plugins when building for UWP

When using GStreamer with Universal Windows Platform apps, dynamic
plugins can only be loaded by filename (without a path) using
gst_plugin_load_file() which will call into g_module_open().

On Windows, GModule calls LoadLibrary() on the filename, but with
UWP we need to use LoadPackagedLibrary() which is basically the same
as LoadLibrary(), except it looks only for DLLs (by name) that have
been packaged as assets with the app.

These assets are not files and cannot be accessed using normal file
APIs such as open() or stat().

The upstream glib merge request for adding LoadPackagedLibrary support
is: https://gitlab.gnome.org/GNOME/glib/merge_requests/951
22 jobs for uwp-arm64-support in 50 minutes and 22 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Preparation
passed #379989
gst indent

00:00:36

passed #379988
manifest

00:00:28

 
  Build
passed #379995
build cerbero android universal

00:11:12

passed #379996
build cerbero cross win32

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passed #379997
build cerbero cross win64

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passed #379994
build cerbero fedora x86_64

00:07:01

manual #379999
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #379998
gst-macos-10.14
build cerbero macos x86_64

00:08:46

passed #379990
build fedora x86_64

00:07:41

passed #379991
build nodebug fedora x86_64

00:05:37

passed #379992
build static fedora x86_64

00:16:01

passed #379993
build static nodebug fedora x86_64

00:11:40

 
  Test
passed #380000
check fedora

00:04:38

passed #380001
integration testsuites fedora

00:09:23

passed #380006
valgrind bad

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passed #380003
valgrind base

00:12:36

passed #380002
valgrind core

00:24:59

passed #380007
valgrind ges

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passed #380004
valgrind good

00:10:04

passed #380005
valgrind ugly

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  Integrate
passed #380008
android universal examples

00:06:47

passed #380009
documentation

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