cmake: Explicitly normalize input paths as they exist on disk

`CollapseFullPath` has two use cases:

1.  Normalize input paths from command-line arguments and environment
    variables.  On Windows, load their on-disk capitalization.

2.  Simplify paths constructed internally.  These should already have
    the correct capitalization.

Audit all `CollapseFullPath` call sites and code paths leading to them.
Replace those calls that normalize input paths with calls to
`ToNormalizedPathOnDisk` to express that they need the on-disk case.

By making this distinction we will later be able to remove the on-disk
capitalization lookup `CollapseFullPath`, thus eliminating disk accesses
from internal path processing.

Issue: #20214