split packages support for GETPKGBUILD

[arenevier]

Split packages are not supported in. For example, running paru -Gp gvim will display the html content of this 404 page

This patch adds support for split packages. In split_repo_aur_pkgbuilds, the base package of the target is checked. If different from the target, the base name will be used instead.

Test: run paru -Gp gvim. It will now display vim PKGBUILD.

No need to do anything for aur. split packages are already supported.

Implementation note:

• Since the base() string lifetime is less than the lifetime of targets, the only way I could find to make it work is to use String instead of &str. I tried to add lifetime annotation for the config parameter, but it was not working out (since it's borrowed in some other place). I am novice in rust. So maybe there is a better way.

• I also modified split_repo_aur_pkgbuilds to return strings instead of the Targ struct. Indeed, the repo field is never used.

[arenevier]

test all pass locally for me. (by running cargo test --features mock)