Unable to compile dependency
### Environment information

* `brownie` Version: 1.11.12 / 3fad0bdc7a65cd1c27d23c0dda4a5276f7255670 (locally)
* `ganache-cli` Version: N/A
* `solc` Version: 0.6.12
* Python Version: 3.8.6
* OS: osx

### What was wrong?
I am trying to add a dependency to another project and I am getting:
```
WARNING: Unable to compile Macarse/GenericKeep3rV2@0.0.1 due to a FileNotFoundError - you may still be able to import sources from the package, but will be unable to load the package directly.
```
Dependency is https://github.com/Macarse/GenericKeep3rV2/releases/tag/0.0.1

Please include information like:
I run brownie locally to find out more.
I added a `print(str(e))` to the warning  and the apparent file missing is: `interfaces/IKeep3rV1.sol`

I kept looking at the brownie code and found the exception happening at:
`changed_sources = {i: self._sources.get(i) for i in changed_paths}` in the `_compile_interfaces` method.
I tried shuffling the `changed_sources` array and all of them seem to be failing.

No idea what the `change_sources` is for.
Any idea what can be wrong?

diff --git a/brownie/project/sources.py b/brownie/project/sources.py
index bb05f7bc..ebf3e434 100644
--- a/brownie/project/sources.py
+++ b/brownie/project/sources.py
@@ -72,14 +72,16 @@ class Sources:
 
         if key in self._contracts:
             return self._contract_sources[self._contracts[key]]
-
-        if key not in self._contract_sources:
-            # for sources outside this project (packages, other projects)
-            with Path(key).open() as fp:
-                source = fp.read()
-                self._contract_sources[key] = source
-
-        return self._contract_sources[key]
+        if key in self._contract_sources:
+            return self._contract_sources[key]
+        if key in self._interface_sources:
+            return self._interface_sources[key]
+
+        # for sources outside this project (packages, other projects)
+        with Path(key).open() as fp:
+            source = fp.read()
+            self._contract_sources[key] = source
+            return source
 
     def get_path_list(self) -> List:
         """Returns a sorted list of source code file paths for the active project."""
['brownie/project/sources.py']