Fix outdated R packages failing to fetch (#11039)

PR #10758 made a slight change to find_versions_of_archive() which included
archive_url in the search process. While this fixed `spack create` and
`spack checksum` missing command-line arguments, it caused `spack
install` to prefer those URLs over those it found in the scrape process.

As a result, the package url was treated as a list_url causing all R
packages to stop fetching once the package was updated on CRAN.

This patch is more selective about including the archive_url in the
remote versions, explicitly overriding it with matching versions found
by the scraper.

lib/spack/spack/util/web.py

@@ -304,9 +304,8 @@ def find_versions_of_archive(archive_urls, list_url=None, list_depth=0):
     list_urls.update(additional_list_urls)
 
     # Grab some web pages to scrape.
-    # Start with any links already given.
     pages = {}
-    links = set(archive_urls)
+    links = set()
     for lurl in list_urls:
         pg, lnk = spider(lurl, depth=list_depth)
         pages.update(pg)
@@ -345,8 +344,10 @@ def find_versions_of_archive(archive_urls, list_url=None, list_depth=0):
         regexes.append(url_regex)
 
     # Build a dict version -> URL from any links that match the wildcards.
+    # Walk through archive_url links first.
+    # Any conflicting versions will be overwritten by the list_url links.
     versions = {}
-    for url in sorted(links):
+    for url in archive_urls + sorted(links):
         if any(re.search(r, url) for r in regexes):
             try:
                 ver = spack.url.parse_version(url)