Add repro-get install-by-name --best-effort PKG1 PKG2 ... to allow installing unpinned packages

[AkihiroSuda]

repro-get install-by-name --best-effort PKG1 PKG2 ... will try to use a pinned package if the pin is present in SHA256SUMS.
If not present, it will just install the available version, and record the pin to the SHA256SUMS file.

[AkihiroSuda]

install-by-name will call these commands:

# `apt-get download` does not download deps
apt-get install --download-only -o Dir::Cache::Archives=/foo gcc

dnf install --downloadonly --destdir /foo gcc

(cd /foo && apk fetch --recursive gcc)

pacman -S --downloadonly --noconfirm --cachedir=/foo gcc