Restore normalization of library category metadata

When the code was switched to using Arduino Lint for release validation, the `metadata.Validate()` function was removed.
At a closer look, it turns out that this function did normalization in addition to validation of the metadata. One such
normalization was to restrict the `category` field value in the library metadata to the allowed values listed in the
Arduino Library Specification, falling back to "Uncategorized" in the event the library has an unsupported value.

Here, the category normalization is restored. Rather than using bespoke code to define the valid categories as before, the
`github.com/arduino/arduino-cli/arduino/libraries` package is used, as in Arduino CLI. This will provide consistency and
reduce maintenance burden.

Author: per1234

go.mod

@@ -3,6 +3,7 @@ module arduino.cc/repository
 go 1.14
 
 require (
+	github.com/arduino/arduino-cli v0.0.0-20210520100059-2666b6ec51e9
 	github.com/arduino/go-paths-helper v1.5.0
 	github.com/arduino/golang-concurrent-workers v0.0.0-20170202182617-6710cdc954bc
 	github.com/go-git/go-git/v5 v5.3.0