feat context: Allow overriding resource set paths

Instead of always inferring the path at which files in a resource set
are located, let users override the path by specifying a `path` field.

This makes it possible to add the same resource set multiple times
with different values while still keeping distinct names for
addressability (for example when using include/exclude).

This fixes #70

context/testdata/explicit-subresource-path.yaml

@@ -0,0 +1,8 @@
+---
+context: k8s.prod.mydomain.com
+include:
+  - name: parent
+    path: parent-path
+    include:
+      - name: child
+        path: child-path