Revert "go/types: enforce Check path restrictions via panics"

This reverts commit b744a11a.

Reason for revert: Broke trybots. (misc-vetall builder is busted)

Change-Id: I651d1c18db2fb3cb6ec12c2ae62024627baf8d77
Reviewed-on: https://go-review.googlesource.com/38332Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/types/api.go

@@ -347,9 +347,6 @@ func (init *Initializer) String() string {
 // file set, and the package path the package is identified with.
 // The clean path must not be empty or dot (".").
 func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
-	if path == "" || path == "." {
-		panic(`path must not be "" or "."`)
-	}
 	pkg := NewPackage(path, "")
 	return pkg, NewChecker(conf, fset, pkg, info).Files(files)
 }