SOURCES and sources in the root dir conflict on case-insensitive filesystems like HFS/APFS.

On a macOS 11.2:

```shell
❯ git clone git@github.com:aws/aws-nitro-enclaves-cli.git
Cloning into 'aws-nitro-enclaves-cli'...
remote: Enumerating objects: 25, done.
remote: Counting objects: 100% (25/25), done.
remote: Compressing objects: 100% (20/20), done.
remote: Total 2772 (delta 4), reused 15 (delta 2), pack-reused 2747
Receiving objects: 100% (2772/2772), 49.28 MiB | 24.71 MiB/s, done.
Resolving deltas: 100% (1887/1887), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

'SOURCES'
'sources'
```

I would recommend renaming one of these (or moving one into a subdir) unless it is critical that they be setup this way.

Inspect the root-level SOURCES and sources paths and search the repository for references to both names. Reproduce the checkout on a case-insensitive macOS filesystem, then determine which rename or relocation preserves the existing references. Done means a fresh clone no longer reports a path collision and the affected tooling still finds its sources.

Written by the indexing model from the issue text.