Package sysdll is an internal leaf package that records and reports which Windows DLL names are used by Go itself. These DLLs are then only loaded from the System32 directory. See Issue 14959.
IsSystemDLL reports whether the named dll key (a base name, like "foo.dll") is a system DLL which should only be loaded from the Windows SYSTEM32 directory.
Filenames are case sensitive, but that doesn't matter because the case registered with Add is also the same case used with LoadDLL later.
It has no associated mutex and should only be mutated serially (currently: during init), and not concurrent with DLL loading.
Add notes that dll is a system32 DLL which should only be loaded from the Windows SYSTEM32 directory. It returns its argument back, for ease of use in generated code.