vppapigen:  move import processing logic to individual plugins

Vppapigen currently embeds the import following control logic in
the runner.  This change delegates the control to the plugins.

Type: refactor

Change-Id: Iad20341bc9d652bedb71ca7037d3957fe60c7a0d
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
diff --git a/src/tools/vppapigen/vppapigen_c.py b/src/tools/vppapigen/vppapigen_c.py
index 07975ce..a879074 100644
--- a/src/tools/vppapigen/vppapigen_c.py
+++ b/src/tools/vppapigen/vppapigen_c.py
@@ -6,6 +6,8 @@
 from io import StringIO
 import shutil
 
+process_imports = False
+
 datestring = datetime.datetime.utcfromtimestamp(
     int(os.environ.get('SOURCE_DATE_EPOCH', time.time())))
 input_filename = 'inputfil'