default: interact with linecache for complete traceback

When using init or install option of slapos.recipe.build to
write python code, the traceback were hard to understand, because
the current line was not included in the traceback.

This put the script in linecache before compiling it, so that in
case of error the problematic line appears in traceback.

There is still the "problem" that the traceback line numbers starts
from the beginning of the script and not the actual line from
buildout profile, but this does not seem feasible to adjust the line
numbers, so let's ignore this.

README.rst

@@ -106,7 +106,8 @@ In case of error, a proper traceback is displayed and nothing is installed
   recipe being used:
   Traceback (most recent call last):
   ...
-    File "<string>", line 3, in <module>
+    File "section-two", line 3, in <module>
+      print(1 / 0.) # this is an error !
   ZeroDivisionError: float division by zero
 
   >>> ls(sample_buildout, 'parts')

slapos/recipe/build/__init__.py

@@ -25,6 +25,7 @@
 #
 ##############################################################################
 import errno
+import linecache
 import logging
 import os
 from platform import uname
@@ -256,7 +257,15 @@ class Script(EnvironMixin):
       g['location'] = options['location']
     except KeyError:
       pass
-    exec(script, g)
+
+    linecache.cache[self.name] = (
+        len(script),
+        None,
+        script.splitlines(),
+        self.name,
+    )
+    code = compile(script, self.name, 'exec')
+    exec(code, g)
 
   def install(self):
     if not self._install: