Skip provider searching for already installed pkgs

We do not offer a provider menu for already installed packages. For
example if you already have yay installed then yay -S yay will just
reinstall yay. No prompt will be given for yay-git or yay-bin.

Therefore it makes sense to disable provider searching for these
packages. Expecially because it is the slowest part of dependency
resolving and many already installed packages are going to be resolved
when using -u.

depPool.go

@@ -203,6 +203,9 @@ func (dp *depPool) findProvides(pkgs stringSet) error {
 	}
 
 	for pkg := range pkgs {
+		if _, err := dp.LocalDb.PkgByName(pkg); err == nil {
+			continue
+		}
 		wg.Add(1)
 		go doSearch(pkg)
 	}