validator/propertyNames: fix instanceLocation and keywordLocation

[truffle-dev]

Fixes #258.

propertyNames validation called the top-level (*Schema).validate entry to check each property name. That builds a fresh root validator (empty vloc) and wraps the result in a kind.Schema error, so the reported instanceLocation was empty instead of the object's location. The caller then overwrote SchemaURL with PropertyNames.Location, and since kind.PropertyNames.KeywordPath() already returns ["propertyNames"], absoluteKeywordLocation appended a second /propertyNames, producing the duplicated keyword path.

What changed

• validator.go: added validatePropertyName, a sub-validator that inherits the current vloc and scope (mirroring validateVal/validateValue), so property-name errors carry the object's instance location and the correct relative keyword path. The propertyNames block now wraps the result via addErrors(causes, &kind.PropertyNames{...}), which builds the parent error at the object's location with the inner failures as causes. The meta-resource swap previously inlined here is handled by the sub-validator's handleMeta().
• validator_test.go: added TestPropertyNamesLocation asserting both the propertyNames error and its cause report instanceLocation /foo and a non-duplicated keywordLocation.

For the issue's example, the output is now:

- at '/foo' [S#/properties/foo/propertyNames]: invalid propertyName 'BAR'
  - at '/foo' [S#/properties/foo/propertyNames/pattern]: 'BAR' does not match pattern '^[a-z]+$'

[truffle-dev]

Rebased onto boon — CLEAN now (80604bd).

4bdda86 fixes the top-level propertyNames instanceLocation, but two things from #258 remain on boon HEAD:

• keywordLocation still doubles to /properties/foo/propertyNames/propertyNames
• the nested keyword (pattern) still reports empty instanceLocation

Basic output for #258's example on boon, keywordLocation → instanceLocation:

/properties/foo/propertyNames/pattern        ""
/properties/foo/propertyNames/propertyNames  /foo

The sub-validator here fixes both — TestPropertyNamesLocation fails on boon and passes on this branch.