impl(language): unexport unnecessary fields

[julieqiu]

Unexport fields that do not need to be accessed outside of the language
package. This revealed several staticcheck and unparam errors, which
are also fixed.

--- FAIL: TestStaticCheck (1.55s)
        internal/language/golang.go:222:19: func (*goCodec).fqEnumValueName is unused (U1000)
--- FAIL: TestUnparam (1.17s)
        internal/language/golang.go:190:47: (*goCodec).messageName - state is unused
        internal/language/golang.go:230:47: (*goCodec).bodyAccessor - state is unused
        internal/language/golang.go:238:48: (*goCodec).httpPathFmt - state is unused
        internal/language/golang.go:252:49: (*goCodec).httpPathArgs - state is unused
        internal/language/rust.go:512:52: (*rustCodec).asQueryParameter - state is unused
        internal/language/rust.go:587:49: (*rustCodec).messageName - state is unused
        internal/language/rust.go:609:43: (*rustCodec).enumName - state is unused
        internal/language/rust.go:631:49: (*rustCodec).bodyAccessor - state is unused
        internal/language/rust.go:639:50: (*rustCodec).httpPathFmt - state is unused
        internal/language/rust.go:696:51: (*rustCodec).httpPathArgs - state is unused

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.51%. Comparing base (a763603) to head (d66f5cb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #670   +/-   ##
=======================================
  Coverage   95.51%   95.51%           
=======================================
  Files          31       31           
  Lines        1181     1181           
=======================================
  Hits         1128     1128           
  Misses         53       53           

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