Fix module specifier case handling for declaration emit

[weswigham]

On case-insensitive filesystems, the EmitHost wasn't propagating the case sensitivity from its parent compiler host, and the types weren't indicating this was a problem because the ModuleSpecifierResolutionHost types are loose to accommodate some odd patterns in the services layer (where some members are optional but we generally expect them to be present if we are to handle things correctly). So while we were handling case correctly in (most) cases in the algorithm itself (I found one place we were doing a === which needed to be a case-aware comparison, but should only have affected the ability to find symlinks), we were never indicating if it was supposed to be case-insensitive in the first place, resulting in paths canonicalized differently within the resolver vs externally to it, with an end result of paths like ../../User/foo/index. (since externally a canonical path like /user/foo/index would be made, but internally we'd compare case-sensitively against the original /User/foo/index).

Fixes #24599

[osdiab]

Excited to see this get released :)