Bugfix: fetching oddly-named resources from local mirrors (#23300)

Spack uses curl to fetch URL resources. For locally-stored resources
it uses curl's file protocol; when using this protocol, curl expects
that the URL encoding conforms to RFC 3986 (which reserves characters
like '?' and '=' for special use).

We were not performing this encoding, and found a resource where
curl was interpreting this in an unfavorable way (succeeding, but
producing an empty file). This commit properly encodes URLs when
using curl's file protocol.

This error did not likely come up before because in most contexts
Spack was either fetching via http or it was using URLs without
offending characters (for example, the sha-based URLs in mirrors
never contain these characters).

lib/spack/spack/fetch_strategy.py

@@ -292,7 +292,15 @@ def mirror_id(self):
 
     @property
     def candidate_urls(self):
-        return [self.url] + (self.mirrors or [])
+        urls = []
+
+        for url in [self.url] + (self.mirrors or []):
+            if url.startswith('file://'):
+                path = urllib_parse.quote(url[len('file://'):])
+                url = 'file://' + path
+            urls.append(url)
+
+        return urls
 
     @_needs_stage
     def fetch(self):