collection of fixes for building without pre-compiled headers

ie, if -DPXR_ENABLE_PRECOMPILED_HEADERS=OFF

wanted to build without precompiled headers to help with debugging a
strange build issue (ie, create more transparent + atomic build steps)

also, turning off PCH usage allows stashed.io (a
caching compiler wrapper for MSVC) to operate when using PDB
(see: https://github.com/playscale/stashed.io/wiki/PDB-and-PCH-generation)

most fixes are pretty self-explanatory - adding some missing headers to
individual source / header files

in stackTrace.cpp, the include of Winsock2.h must happen before windows.h,
which is brought in via fileSystem.h

See:
PixarAnimationStudios/OpenUSD#1672

stackTrace.cpp

@@ -32,14 +32,18 @@
 #include "pxr/base/arch/error.h"
 #include "pxr/base/arch/errno.h"
 #include "pxr/base/arch/export.h"
+#if defined(ARCH_OS_WINDOWS)
+// Need to include Winsock2.h BEFORE windows.h - which is included in
+// fileSystem.h
+#include <Winsock2.h>
+#endif
 #include "pxr/base/arch/fileSystem.h"
 #include "pxr/base/arch/inttypes.h"
 #include "pxr/base/arch/symbols.h"
 #include "pxr/base/arch/vsnprintf.h"
 #if defined(ARCH_OS_WINDOWS)
 #include <io.h>
 #include <process.h>
-#include <Winsock2.h>
 #include <DbgHelp.h>
 #ifndef MAXHOSTNAMELEN
 #define MAXHOSTNAMELEN 64