Add custom structfield linter to check struct field names and tags

[alexandear]

This PR renames jsonfieldname linter to structfield and extends it to check struct tags.

The linter enforces that:

• The tag names for json and url match the struct field name
• Fields with json and url tags that include omitempty must use pointers types, except for slices, maps, json.RawMessage, and any.

Example output for the ListCursorOptions struct:

go-github/github/github.go

Lines 262 to 267 in 671b8b0

	type ListCursorOptions struct {
	// For paginated result sets, page of results to retrieve.
	Page string `url:"page,omitempty"`
	// For paginated result sets, the number of results to include per page.
	PerPage int `url:"per_page,omitempty"`

$ ./script/lint.sh
github/github.go:264:7: change the "Page" field type to "*string" in the struct "ListCursorOptions" because its tag uses "omitempty" (structfield)
        Page string `url:"page,omitempty"`
             ^
github/github.go:267:10: change the "PerPage" field type to "*int" in the struct "ListCursorOptions" because its tag uses "omitempty" (structfield)
        PerPage int `url:"per_page,omitempty"`
                ^
...

Closes #3775

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (903265b) to head (e83e366).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3843   +/-   ##
=======================================
  Coverage   92.41%   92.42%           
=======================================
  Files         197      197           
  Lines       14152    14167   +15     
=======================================
+ Hits        13079    13094   +15     
  Misses        884      884           
  Partials      189      189           

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[gmlewis]

Cool! Thank you, @alexandear!
Just a couple questions, really.

[gmlewis]

Does this linter also handle the following cases? If so, could you please demonstrate them in this file?

• *[]string => []string
• *[]anythingReally => []anythingReally
• *map[one]two => map[one]two
• []*string => []string
• []*int<64> => []int<64>

[alexandear]

Thanks for review. Added these cases, please check.

[gmlewis]

So I think I confused myself with the sliceofpointers linter with the above comment.
Does this linter now have all the functionality of the sliceofpointers linter, and do we need it anymore now that this one is updated?

[alexandear]

structfield analyzes only struct fields, while sliceofpointers checks variables. We could combine both linters, but we’d need to come up with a better, more general name for the merged version.

[gmlewis]

Could you please also demonstrate that []*int64 and []*bool and []*float64 also get flagged properly since those are probably going to be the most common after int?