Do not resolve relative external ids when using the object form

[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:
Resolves #2867

Description

This change will improve the behaviour of the resolveId hook so that the user has the option to specify external ids that will appear in the generated bundle unmodified even in the case of relative imports. This also contains a documentation update that explains this.

The logic is now:

• if a relative import is marked as external via user option or by returning "false" from the "resolveId" hook, Rollup will internally resolve the import to an absolute file system location to be able to merge different relative imports of the same external file. When the bundle is written, the absolute location will be converted to a relative import again as if the "output.file" or "output.dir" option were in the same location as the entry point (or the common base directory of all entry points) (existing behaviour).
• if "resolveId" returns an object with an absolute id and "external: true", this id will be resolved to a relative import in the generated bundle (existing behaviour).
• if "resolveId" returns an object with a relative id and "external: true", this id will not be resolved but used in the generated bundle unmodified (new behaviour)