Sanitize chunk names to allow for virtually created chunks

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

This will allow plugins to use virtual modules (i.e. starting with a \0) as entry points. This will enable rollup-plugin-multi-entry to work with rollup@1.0 as it creates such a virtual entry point,.

[guybedford]

It could be good to ensure that chunkAlias is sanitized at the time it is set on the modules, instead of at this stage. Strictly speaking, we should probably throw for invalid scenarios over sanitizing as this is user input? But it's very much a nit.

[lukastaegert]

In fact no, we should not sanitize the id on the module. The reason is that by convention, some plugins (such as rollup-plugin-multi-entry) create virtual modules with an id that starts with a \0 to signal that other plugins should ignore this module for e.g. loading and transformation. Thus we need to keep the module id unsanitized to not break plugins. The chunk id, however, is what is written to disk and is something we can choose rather freely.