Import some fixes from resolver branch.

ROADMAP

@@ -0,0 +1,44 @@
+class UpstreamRequirement(object):
+
+    family: str
+
+
+class PythonPackageRequirement(UpstreamRequirement):
+
+    package: str
+
+
+SetupPy.get_build_requirements() yields some PythonPackageRequirement objects
+
+apt_resolver.install([PythonPackageRequirement(...)]) then:
+
+ * needs to translate to apt package name
+
+
+Once we find errors during build, buildlog consultant extracts them ("MissingPythonPackage", "configure.ac needs updating").
+
+fix_build then takes the problem found and converts it to an action:
+
+ * modifying some of the source files
+ * resolving requirements
+
+Resolving requirements dependencies means creating e.g. a PythonPackageRequirement() object and feeding it to resolver.install()
+
+we have specific handlers for each kind of thingy
+
+resolver.install() needs to translate the upstream information to an apt name or a cpan name or update dependencies or raise an exception or..
+
+MissingPythonPackage() -> PythonPackageRequirement()
+
+PythonPackageRequirement() can either:
+
+ * directly provide apt names, if they are known
+ * look up apt names
+
+We specifically want to support multiple resolvers. In some cases a resolver can't deal with a particular kind of requirement.
+
+Who is responsible for taking a PythonPackageRequirement and translating it to an apt package name?
+
+ 1) PythonPackageRequirement itself? That would mean knowledge about package naming etc, is with the requirement object, which seems wrong.
+ 2) PythonPackageRequirement.apt_name(apt_archive) - i.e. find the package name given an archive object of some sort
+ 3) The apt resolver has a list of callbacks to map requirements to apt package names