scons: Move the source related helper classes out of src/SConscript.

By having them in gem5_scons.sources, they can be used by mechanisms
outside of src/SConscript, like separated out builders.

Change-Id: Ic3769723c8413e7db48aef536572ad3f2f948658
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48376
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

SConstruct

@@ -130,6 +130,7 @@ from gem5_scons import error, warning, summarize_warnings, parse_build_path
 from gem5_scons import TempFileSpawn, EnvDefaults, MakeAction, MakeActionTool
 import gem5_scons
 from gem5_scons.builders import ConfigFile, AddLocalRPATH, SwitchingHeaders
+from gem5_scons.sources import TagImpliesTool
 from gem5_scons.util import compareVersions, readCommand
 
 # Disable warnings when targets can be built with multiple environments but
@@ -149,7 +150,7 @@ Export('MakeAction')
 
 main = Environment(tools=[
         'default', 'git', TempFileSpawn, EnvDefaults, MakeActionTool,
-        ConfigFile, AddLocalRPATH, SwitchingHeaders
+        ConfigFile, AddLocalRPATH, SwitchingHeaders, TagImpliesTool
     ])
 
 main.Tool(SCons.Tool.FindTool(['gcc', 'clang'], main))