The IPDL compiler searches for files in an include path made up of every
directory with any `IPDL_SOURCES` in it, which means that distinct files
shouldn't have the same name or, failing that, need to be set up so they
won't accidentally conflict. See bug 1657712 for details.
In `widget/`, the PCompositorWidget and PlatformWidgetTypes files are
overridden on some OSes; this patch moves the general-case versions into
their own directory, so that it's possible to use the widget/ toplevel
directory for non-platform-specific IPDL without causing a filename
conflict.
Differential Revision: https://phabricator.services.mozilla.com/D94530