Upgrade crates.io URL from "api/v1" to "static" in purl2url

[tdruez]

Following aboutcode-org/dejacode#563

The packageurl-python library currently return https://crates.io/api/v1/ URLs in the purl2url function.

purl = "pkg:cargo/allocator-api2@0.2.21"
purl2url.get_download_url(purl)
'https://crates.io/api/v1/crates/allocator-api2/0.2.21/download'

Looking at https://crates.io/data-access

you are welcome to use the crates.io API provided you abide by the following limits:

1. A maximum of 1 request per second, and
2. A user-agent header that identifies your application.

Since the https://static.crates.io/ type URL have no rate limits at present, it could be an improvement for purl2url to return such URL:

https://static.crates.io/crates/allocator-api2/allocator-api2-0.2.21.crate