Underscore and dash should be equivalent in haxelib names

[tobil4sk]

Since the haxe compiler treats -D some-lib and -D some_lib as equivalent, it might make sense to treat these as equivalent library names. Otherwise, two different libraries could set the same define flag.

[tobil4sk]

I realise now that there's a lot more problems with library name flags:

• Library names are case insensitive, but compiler flags aren't. I think in the current development branch it always uses what's in haxelib.json if available, or otherwise, whatever name the library was installed as.
• Library names could shadow actual compiler flags. It wouldn't really be pratical to have a list of banned library names, and that won't work for future compiler flags. Luckily, there only seems to be one instance currently, which is hxcpp-debugger which doesn't seem to be too serious.

Ideally they would be put in a haxelib namespace, like haxelib.some_lib. However, that would be a difficult migration. It would be possible to potentially have warning messages if an #if block checks for some_lib which is not set, and there exists a haxelib.some_lib flag.