go/doc,x/pkgsite: struct fields inconsistently linked in doc comments

[bcmills]

https://go.dev/doc/comment#doclinks currently says:

Doc links are links of the form “[Name1]” or “[Name1.Name2]” to refer to exported identifiers in the current package

but it is somewhat ambiguous about whether the [Name1.Name2] form can refer to struct fields or only methods.

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In practice, people do seem to intuitively expect references to struct fields to work; for example, this comment on types.Package.GoVersion (written by @rsc) refers to the go/ast.File.GoVersion struct field, and pkg.go.dev appears to render that reference correctly:

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However, similar references within the same package are not rendered as links. For example, the reference to [CheckedFiles.SizeError] from zip.CheckedFiles.Err is currently rendered as plain text:

go/doc.Parser uses different functions to resolve intra- vs. inter-package references: it appears that the LookupPackage hook being used by pkgsite does resolve struct fields, whereas the LookupSym hook does not.

[bcmills]

I think the bug is in go/doc. Although the Package.syms field lacks a clarifying comment, in practice it appears not to include struct fields:
https://cs.opensource.google/go/go/+/refs/heads/master:src/go/doc/doc.go;l=159-171;drc=fe5af1532ab9c749d880c05e0ffe0e17bf874d7f

[gopherbot]

Change https://go.dev/cl/510315 mentions this issue: go/doc: recognize intra-package struct field references

[gopherbot]

Change https://go.dev/cl/528402 mentions this issue: go/doc: add a golden test that reproduces #62640