Add trailing slash when spidering URLs for versions (#8429)

By default, if a package does not specify a list_url and does not download from
a common repository, Spack runs dirname on the package URL. Given a URL
like https://root.cern.ch/download/root_v6.09.02.source.tar.gz, this returns
https://root.cern.ch/download. However, https://root.cern.ch/download
gives a 404, while https://root.cern.ch/download/ works just fine.

Note that some servers *don't* work with a trailing slash, so this tries with and
without the slash. This will double the number of URLs searched but the
slowdown should only affect the "spack versions" command.

lib/spack/spack/util/web.py

@@ -286,6 +286,13 @@ def find_versions_of_archive(archive_urls, list_url=None, list_depth=0):
     for aurl in archive_urls:
         list_urls.add(spack.url.find_list_url(aurl))
 
+    # Add '/' to the end of the URL. Some web servers require this.
+    additional_list_urls = set()
+    for lurl in list_urls:
+        if not lurl.endswith('/'):
+            additional_list_urls.add(lurl + '/')
+    list_urls.update(additional_list_urls)
+
     # Grab some web pages to scrape.
     pages = {}
     links = set()