Use file paths/urls correctly (#34452)

The main issue that's fixed is that Spack passes paths (as strings) to
functions that require urls. That wasn't an issue on unix, since there
you can simply concatenate `file://` and `path` and all is good, but on
Windows that gives invalid file urls. Also on Unix, Spack would not deal with uri encoding like x%20y for file paths. 

It also removes Spack's custom url.parse function, which had its own incorrect interpretation of file urls, taking file://x/y to mean the relative path x/y instead of hostname=x and path=/y. Also it automatically interpolated variables, which is surprising for a function that parses URLs.

Instead of all sorts of ad-hoc `if windows: fix_broken_file_url` this PR
adds two helper functions around Python's own path2url and reverse.

Also fixes a bug where some `spack buildcache` commands
used `-d` as a flag to mean `--mirror-url` requiring a URL, and others
`--directory`, requiring a path. It is now the latter consistently.

share/spack/spack-completion.bash

@@ -556,7 +556,7 @@ _spack_buildcache_sync() {
 }
 
 _spack_buildcache_update_index() {
-    SPACK_COMPREPLY="-h --help -d --mirror-url -k --keys"
+    SPACK_COMPREPLY="-h --help -d --directory -m --mirror-name --mirror-url -k --keys"
 }
 
 _spack_cd() {