New mix compiler may error after partial project compilation

[ericmj]

Steps to reproduce:

1. Checkout this branch on ecto: https://github.com/elixir-lang/ecto/compare/mix-compile
2. mix clean && mix compile
3. Note that elixir compilation fails, as expected, on this: https://github.com/elixir-lang/ecto/compare/mix-compile#diff-e10170ac8baa40c81319d2956167adcdR83
4. The project is now partially compiled, remove the failing line and comment out the line above
5. mix compile
6. Compilation will now fail with the following error message:

[snip]
== Compilation failed ==
Compilation failed on the following files:

* lib/ecto/associations.ex is missing module Ecto.Query
* lib/ecto/repo/backend.ex is missing module Ecto.Query

The first failure is shown below...
== Compilation error on file lib/ecto/repo/backend.ex ==
** (CompileError) lib/ecto/repo/backend.ex:11: module Ecto.Query is not loaded and could not be found
[snip]