Add ability to merge matches

[wagoodman]

Primarily this PR adds the ability to merge two matches that have the same vulnerability-package pair, but the matching details may be different (what was searched by, what attribute was matched on, the matcher that found the result, etc). In the case of merging the list of match.Details is combined and one match object is kept.

This PR additionally makes the following adjustments:

• updates tests fixtures that use package objects to have package IDs
• updates the match.NewMatches() constructor to take an initial variadic list of matches
• renames/moves the matchers.common package to search which makes usage less awkward and more semantically correct (the matchers use search utilities, not common utilities... e.g. common.FindMatchesByPackageLanguage(...) --> search.ByPackageLanguage(...))
• adds search.ByCriteria to reduce the code for each matcher object. This turns the old approach:

	var matches = make([]match.Match, 0)
	langMatches, err := common.FindMatchesByPackageLanguage(store, p.Language, p, m.Type())
	if err != nil {
		return nil, err
	}
	matches = append(matches, langMatches...)

	cpeMatches, err := common.FindMatchesByPackageCPE(store, p, m.Type())
	if err != nil {
		return nil, err
	}
	matches = append(matches, cpeMatches...)
        return matches, nil

into:

       return search.ByCriteria(store, d, p, m.Type(), search.ByLanguage, search.ByCPE)

This work is loosely related to #395

[spiffcs]

No nits or large comments. This is a huge upgrade that people can start using immediately both within the binary and when they consume grype as a library