Do not create conflicts between files emitted in generateBundle for different outputs

[lukastaegert]

• Rollup Version: 1.20.3
• Operating System (or Browser): Any
• Node Version: 12

How Do We Reproduce?

Taken from #2999 (comment):
Note that I did not verify this myself, but I do not want this to go unnoticed cc @wessberg

Expected Behavior

When creating several outputs and emitting files in generateBundle using the fileName option, the files are unique to each output and there are not conflicts between outputs.

Actual Behavior

Apparently, files in later outputs cannot be generated with the same name but will throw a name conflict error.

[wessberg]

This issue was resolved in #3174 and released in v1.26.1.

[lukastaegert]

Unfortunately not completely. You can still create conflicts via the JS API. But the real fix is around the corner.

[wessberg]

For what it's worth, the problem I initially reported which involved using the emitFile API went away with v1.26.1. Keep up the good work Lukas, you're awesome! 👍