getDTName logic is brittle

The current logic in https://github.com/microsoft/DefinitelyTyped-tools/blob/HEAD/packages/dts-gen/src/names.ts#L1 is basically "if it's a scoped package, replace the slash with __".

Unfortunately, this fails in the event of an existing package with a double underscore. I don't have a concrete existing example, so it's not an urgent problem :-) but, eg the types for @eslint/js are in @types/eslint__js, but anyone could make a package eslint__js, and then what would the types package for that be?

Obviously changing existing DT package names would be very disruptive, but if we could come up with, for non-scoped packages that have a double underscore, a new convention, then that'd close this theoretical hole (even if nobody ever needed it).

One idea is eslint__js → eslint____js, eg replace __ with ____. this would mean that eslint____js (4 underscores) would become 8, etc, but the likelihood of this running into package name limits is even lower than the likelihood of 2 underscores being used.

Essentially, any replacement scheme would make __ in a regular package name be transformed into something else, and that would require a similar cascade as above.

Start in packages/dts-gen/src/names.ts at the getDTName logic. Determine a naming convention that distinguishes scoped packages from unscoped packages containing double underscores while preserving existing DefinitelyTyped package names. Done means the convention resolves the described collision cases without requiring disruptive renames.