ConvertService relies on documentation extension files being considered "resolved" even when build has no symbols

[d-ronnqvist]

Description

The ConvertService relies on an old implementation detail where every documentation extension file was always “resolved” even when it didn’t match a symbol or when the build had no symbol graph files.

This behavior is different from how documentation extension files work in regular builds. Some consequences of this are:

• the process that interacts with the convert service can't distinguish between documentation extension files that match symbols and documentation extension files that don't match symbols.
• the process that interacts with the convert service is responsible for maintaining it's own link resolutions implementation to match the behavior of a regular build.
• the process that interacts with the convert service is responsible for diagnosing documentation extension files that don't matching symbols and for matching the diagnostics that would be in a regular build.

Checklist

• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue.

Expected Behavior

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Actual behavior

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Steps To Reproduce

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Swift-DocC Version Information

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Swift Compiler Version Information

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