Module rename

[BCSharp]

Currently submodule DLR uses the following three "names":

1. URL, from which it is being pulled: https://github.com/IronLanguages/dlr
2. The directory path, to which it is being pulled into: src/dlr
3. The module name itself, currently "Src/DLR"

The last one is mainly used by Git internal commands, but is being defined in .gitmodules. This is almost the same as the path, except for the letter case. This is no problem for Windows and macOS, but can be a problem on Linux. To avoid confusion, this PR normalizes the submodule name to simply "DLR". There is one module in IronPython3 after all, and there is no need to create a hierarchy.

To avoid problems with update, I recommend, on a clear DLR worktree, on the branch main:

1. recursively copy content of directory .git/modules/Src/DLR to directory .git/modules/DLR (if using in a linked worktree, replace .git with $(cut -d' ' -f 2 .git), Bash syntax, only if linked worktree uses a separate config. Pwsh syntax: ((sls 'gitdir:' .git) -split '\s+')[1])
2. deinit the submodule: git submodule deinit --all
3. update the main branch from upstream (i.e. IronLanguages/ironpython3): git pull upstream main
4. reinit the submodule: git submodule init
5. update the submodule: git submodule update

This PR also includes some path changes needed for the submodule to work in IronPython.

[slozier]

Thanks!