Implement file-based fetcher

Thanks for making this into a patch (https://www.drupal.org/node/2779083)...

As I contemplate #2802447: Fetcher plugin for sftp, given there are stream wrappers for sftp and related protocols, I look here and see file_get_contents, which of course can read from any stream wrapper. I think with just a little tweaking, this plugin can work generally with simple stream wrappers, and then we only need to use specialized fetchers like http when we need to pass options beyond a simple URL. And assuming this does work for local files/HTTP/etc., it should be worth making this the default fetcher for the url source plugin, so it won't need to be specified.

Since I need this for my current project, I'll pick it up from here today/tomorrow.

Here we go - I've tested this version of the fetcher plugin with a local absolute path, a path relative to the Drupal root, an http URL, and (my main motivation) a ssh2.sftp URL. Seems to work fine...

Committed with a bit more tweaking, thanks!

#6 works for me, thanks!

1. +++ b/src/Plugin/migrate_plus/data_fetcher/File.php
   @@ -0,0 +1,58 @@
   +  public function setRequestHeaders(array $headers) {
   +    // Does nothing.
   ...
   +  public function getRequestHeaders() {
   +    // Does nothing.
   +    return [];
   

   At this point, maybe it doesn't matter, but why are we requiring no-op methods like these in the data fetcher interface? Maybe that's for a follow-up.

2. +++ b/src/Plugin/migrate_plus/data_fetcher/File.php
   @@ -0,0 +1,58 @@
   +      throw new MigrateException('Error message: ' . $e->getMessage() . ' at ' . $url .'.');
   

   Nit: the doxygen doesn't mention that it can throw an exception. Is that something we want to include in its docs?

3. This should also have tests, which should be easy enough.

At this point, maybe it doesn't matter, but why are we requiring no-op methods like these in the data fetcher interface? Maybe that's for a follow-up.

Yep, going to follow-up by finally dealing with #2640512: Make custom headers an HTTP-specific feature.

Nit: the doxygen doesn't mention that it can throw an exception. Is that something we want to include in its docs?

Good point for a quickfix.

This should also have tests, which should be easy enough.

Everything needs tests:P.