Support modules with different casing in build backend (#12240)
Match the module name to its module directory with potentially different
casing.
For example, a package may have the dist-info-normalized package name
`pil_util`, but the importable module is named `PIL_util`.
We get the module name either as dist-info-normalized package name, or
explicitly from the user. For dist-info-normalizing a package name, the
rules are lowercasing, replacing `.` with `_` and replace `-` with `_`.
Since `.` and `-` are not allowed in module names, we can check whether
a directory name matches our expected module name by lowercasing it.
Fixes #12187
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
