mesa: move software texel fetch code into swrast

It's only used by swrast now so move it out of core Mesa.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index dfc8bd4..ff1ffe0 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -114,7 +114,6 @@
     'main/texcompress_s3tc.c',
     'main/texcompress_fxt1.c',
     'main/texenv.c',
-    'main/texfetch.c',
     'main/texformat.c',
     'main/texgen.c',
     'main/texgetimage.c',
@@ -173,6 +172,7 @@
     'swrast/s_span.c',
     'swrast/s_stencil.c',
     'swrast/s_texcombine.c',
+    'swrast/s_texfetch.c',
     'swrast/s_texfilter.c',
     'swrast/s_texrender.c',
     'swrast/s_texture.c',