Fix PdlStorage Link Handling

Reference: https://code.usgs.gov/ghsc/hazdev/earthquake-event-ws/-/blob/master/src/lib/classes/pdl/PdoProductStorage.class.php#L130-134

Current implementation creates an empty linksMap object and sets that object on the product. This results in any Json Product that is parsed by the PdoProductStorage class to have no links associated with it. Luckily, links are not widely (i.e., ever) used at this time.

I think inside the for-loop, the linksMap should be getting updated rather than updating the links array itself. Otherwise, the implementation is correct.

The idea behind this chunk of code is to map the links from a list of objects with relation and uri keys to a dictionary keyed by relation pointing to a list of uris. For example:

// Input links
links = [
  {"relation": "relation_one", "uri": "uri_A_for_relation_one"},
  {"relation": "relation_one", "uri": "uri_B_for_relation_one"},
  {"relation": "relation_two", "uri": "uri_C_for_relation_two"},
];

// Resulting linksMap
linksMap = {
  "relation_one": [
    "uri_A_for_relation_one",
    "uri_B_for_relation_two",
  ],
  "relation_two": [
    "uri_C_for_relation_two",
  ],
};

// Then set the linksMap on the product