bug#40893: import crate: Recursive importer loops

[Hartmut Goebel]

When running "import crate -r", the same packages get downloaded again
and again. Depending on the package to be imported, this looping can
take quite some time and the user gets the impression, that the import
will recurse endlessly.

I would expect the importer to recognize it did already download the
package.

$ guix pull --commit=ee8de7381  # to be sure the packages are not
imported already.
$ ./pre-inst-env guix import crate -r h2
...
following redirection to
`https://static.crates.io/crates/rustls/rustls-0.17.0.crate'...
...
following redirection to
`https://static.crates.io/crates/webpki/webpki-0.21.2.crate'...
following redirection to
`https://static.crates.io/crates/webpki-roots/webpki-roots-0.19.0.crate'...
...
following redirection to
`https://static.crates.io/crates/webpki/webpki-0.21.2.crate'...
following redirection to
`https://static.crates.io/crates/webpki-roots/webpki-roots-0.19.0.crate'...
...
following redirection to
`https://static.crates.io/crates/ring/ring-0.17.0-alpha.1.crate'...
following redirection to
`https://static.crates.io/crates/ring/ring-0.17.0-alpha.1.crate'...
...
following redirection to
`https://static.crates.io/crates/webpki/webpki-0.21.2.crate'...
...
following redirection to
`https://static.crates.io/crates/rustls/rustls-0.17.0.crate'...
following redirection to
`https://static.crates.io/crates/webpki/webpki-0.21.2.crate'...
following redirection to
`https://static.crates.io/crates/webpki-roots/webpki-roots-0.19.0.crate'...

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | address@hidden               |
| www.crazy-compilers.com | compilers which you thought are impossible |