[bug] export_sources fails if a local repo does not contain the recipe revision being exported

[cynix]

Describe the bug

In a setup where a local repo has higher priority than a remote repo, and a particular recipe revision only exists in the remote repo, operations that involve export_sources for this recipe will fail rather than downloading the sources from the remote.

How to reproduce it

• Add an empty folder as a local repo at index 0, and ensure conancenter is enabled at index 1.
• Run conan create with a conanfile that requires any package from the conancenter repo.