gopls/hover: show named func type doc when hovering over func literal

[abhay1999]

When a func literal is implicitly converted to a named function type
(e.g. fs.WalkDirFunc), hovering over its "func" keyword now shows the
documentation and signature of that named type, with a link to
pkg.go.dev.

Previously, hovering over "func" in:

filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
	...
})

showed only the anonymous signature. Now it shows the docs for
fs.WalkDirFunc, which is often the information the user actually wants.

Implementation: when cur.Node() is *ast.FuncType with the cursor at the
"func" token and the parent is a *ast.FuncLit, the new hoverFuncLit
function calls typesutil.TypesFromContext to find the named function
type the literal must satisfy, loads the declaring package via
NarrowestDeclaringPackage, and returns a hoverResult with the named
type's documentation, signature, and pkg.go.dev link. If no named type
is found, it falls back to the existing anonymous-type hover behavior.

Note: FindByPos at a FuncLit's "func" token returns the inner
*ast.FuncType node (not *ast.FuncLit), so the code checks the parent
node to detect the FuncLit context.

A marker test is added in hover/funclittype.txt covering both the
named-type case and the fallback case.

Fixes golang/go#76191

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Abhay Chaurasiya:

Patch Set 1:

(1 comment)

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Message from Hongxiang Jiang:

Patch Set 2: Commit-Queue+1

(3 comments)

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Patch Set 2:

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Patch Set 2: -Commit-Queue

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/x_tools-gotip-linux-arm64-boringcrypto has failed with summary (view all results):

---

• go on master (commit d987c23) / tools on master (change 764120)

To reproduce, try gomote repro 8680799669376353153.

Additional links for debugging:

• test golang.org/x/tools module output
• test golang.org/x/tools/gopls module output

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Patch Set 2: LUCI-TryBot-Result-1

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Message from Hongxiang Jiang:

Patch Set 2: Commit-Queue+1

(2 comments)

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Patch Set 2:

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Message from Hongxiang Jiang:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_3494492>)

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/x_tools-gotip-linux-arm64 has failed with summary (view all results):

---

• go on master (commit d987c23) / tools on master (change 764120)

To reproduce, try gomote repro 8680798878888439249.

Additional links for debugging:

• test golang.org/x/tools module output
• test golang.org/x/tools/gopls module output

---

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After addressing review feedback, remember to publish your drafts!