repos: Add candidates_to_build

This is a helper function to find out which packages
from one repo can probably be immediately build for
the other one.

The implementation is suboptimal, but works just fine
for now.

repos_cmp/repos.py

@@ -765,6 +765,34 @@ def extras_to_remove(base_repo, repo, ignore=()):
         print(name.decode())
 
 
+def read_list(filename):
+    '''Read list of names from a file'''
+    with open(filename, 'rb') as f:
+        return f.read().split()
+
+
+def write_list(data, filename):
+    '''Write a list of name to the file'''
+    with open(filename, 'wb') as f:
+        f.write(b'\n'.join(data))
+        f.write(b'\n')
+
+
+def candidates_to_build(build_reporter,
+                        ignore=(),
+                        ignore_jpp=False):
+    ignore = set(ignore)
+    ignore.update(build_reporter.to_repo.sources)
+    for name, src in build_reporter.from_repo.sources.items():
+        if name in ignore:
+            continue
+        if ignore_jpp and b'jpp' in src.release:
+            continue
+        if build_reporter.report(name).triplets:
+            continue
+        yield name
+
+
 def _format_evr(pkg):
     if not pkg:
         return 'MISSING'