Fixes duplicate key error in YAML marshalling

[thoma01]

Found when using import-beats script from elastic/integrations. elastic/integrations#1876

panic: Duplicated key 'categories' in struct util.Package [recovered]
        panic: Duplicated key 'categories' in struct util.Package

goroutine 1 [running]:
gopkg.in/yaml%2ev2.handleErr(0xc001748c98)
        C:/Users/Administrator/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:249 +0x6d
panic({0x159bc60, 0xc0000360f0})
        C:/Program Files/Go/src/runtime/panic.go:1038 +0x215
gopkg.in/yaml%2ev2.(*encoder).structv(0xc0009b0b00, {0x0, 0x0}, {0x15f88a0, 0xc00004e000, 0x0})
        C:/Users/Administrator/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/encode.go:211 +0xe9
gopkg.in/yaml%2ev2.(*encoder).marshal(0xc0009b0b00, {0x0, 0x0}, {0x15f88a0, 0xc00004e000, 0x34})
        C:/Users/Administrator/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/encode.go:160 +0x9a5
gopkg.in/yaml%2ev2.(*encoder).marshalDoc(0xc0009b0b00, {0x0, 0x0}, {0x15f88a0, 0xc00004e000, 0xc001748cb8})
        C:/Users/Administrator/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/encode.go:93 +0x125
gopkg.in/yaml%2ev2.Marshal({0x15f88a0, 0xc00004e000})
        C:/Users/Administrator/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:203 +0x37e
main.(*packageRepository).save(0xc001749d98, {0x160f982, 0x8})
        C:/git/integrations/dev/import-beats/packages.go:269 +0x235
main.build({{0x160f52a, 0x8}, {0x1616cd9, 0x15}, {0x160f292, 0x7}, {0x160f84a, 0x8}, {0x160fc20, 0x9}, ...})
        C:/git/integrations/dev/import-beats/main.go:139 +0x2df
main.main()
        C:/git/integrations/dev/import-beats/main.go:102 +0x4b8
exit status 2

This PR resolves this error.

[elasticmachine]

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