BBHandler: Drop cwd from search path

Whilst bitbake has done this for a long time, the behaviour of resolving class files against cwd is not desirable. This can be seen during base configuration parsing when looking for base.bbclass where a dependency on cwd is added. If cwd then changes, the cache is invalid and triggers a re-parse.

The only real option is to drop this entry and if files can't be found, we fix BBPATH in the cases where it needs fixing. I didn't find any in the random selection of layers I tested parsing locally.

index 56d4672..85c27c2 100644
@@ -76,8 +76,7 @@ def inherit(files, fn, lineno, d):
             file = os.path.join('classes', '%s.bbclass' % file)
 
         if not os.path.isabs(file):
-            dname = os.path.dirname(fn)
-            bbpath = "%s:%s" % (dname, d.getVar("BBPATH", True))
+            bbpath = d.getVar("BBPATH", True)
             abs_fn, attempts = bb.utils.which(bbpath, file, history=True)
             for af in attempts:
                 if af != abs_fn: