dds/etc1/pbm/pvr: Make those modules and split thirdparty files

They are not particularly packaged in Linux distros so we do not
facilitate unbundling via SCons. There could be done if/when there
is interest.

Also s/pnm/pbm/, long-lived typo :)

modules/dds/SCsub

@@ -0,0 +1,7 @@
+Import('env')
+Import('env_modules')
+
+env_modules.add_source_files(env.modules_sources, "*.cpp")
+
+Export('env_modules')
+Export('env')