Handle %(__buildout_space_n__)s & co in .installed.cfg

For example here is how it occurs in .installed.cfg for ERP5 SR:

    [jupyter]
    eggs = jupyter
            jupyter_client
            jupyter_console
            ...
            jupyterlab
            jupyterlab-launcher%(__buildout_space_n__)s

and without the fix it breaks as

    configparser.InterpolationMissingOptionError: Bad value substitution: option 'eggs' in section 'zzz' contains an interpolation key '__buildout_space_n__' which is not a valid option name. Raw value: '\naaa\nbbb%(__buildout_space_n__)s'

nxdbom/__init__.py

@@ -72,7 +72,14 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
             bom[bkey] = info
 
 
-    idb = configparser.ConfigParser()
+    _ = {
+        '__buildout_space__':   ' ',
+        '__buildout_space_n__': '\n',
+        '__buildout_space_r__': '\r',
+        '__buildout_space_f__': '\f',
+        '__buildout_space_v__': '\v',
+    }
+    idb = configparser.ConfigParser(defaults=_)
     idb.read('%s/.installed.cfg' % installed_software_path)
 
 
@@ -157,6 +164,8 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
             eggdev = part['_d']
             eggs = part.get('eggs', part.name).split('\n')
             for eggname in eggs:
+                if eggname == '':   # happens when %(__buildout_space_n__)s is appended  to eggname
+                    continue        # because software.cfg contained vspace
                 m = _egg_re.match(eggname)
                 assert m is not None, eggname
                 eggname = m.group('name')   # neoppod[admin, ctl, master] -> neoppod

nxdbom/nxdbom_test.py

@@ -225,6 +225,23 @@ msgpack                      0.5.4      http://downloads.sourceforge.net/project
 msgpack                      0.6.2      https://pypi.org/project/msgpack/0.6.2/
 """)
 
+# %(__buildout_space_...) in egg can be read
+case1("""\
+[zzz]
+recipe = zc.recipe.egg
+_d = /ROOT/develop-eggs
+_e = /ROOT/eggs
+eggs =
+    aaa
+    bbb%(__buildout_space_n__)s
+-- /ROOT/eggs/aaa-1.2.3.egg/x --
+-- /ROOT/eggs/bbb-5.6.7.egg/x --
+""", """
+>>> eggs:
+aaa                          1.2.3.egg  https://pypi.org/project/aaa/1.2.3.egg/
+bbb                          5.6.7.egg  https://pypi.org/project/bbb/5.6.7.egg/
+""")
+
 # %20 in URL
 case1("""\
 [zabbix-agent]