avoid downloads where possible

[ThomasBreuer]

When InstallPackage is called with a package name and a version then currently PackageManager first downloads the current PackageInfo.g data.
We should first check whether the requested version of the package is already installed, and immediately return true if yes.

(This idea is proposed for the Julia package GAP.jl in oscar-system/GAP.jl/pull/1019, but the natural place for it is in PackageManager.)

[mtorpey]

This problem goes away with the move to the package distro – one download of metadata per session, and the rest is just actual downloads of package archives.

[ThomasBreuer]

Not really.
The point is that no download is needed at all if the requested version is already installed.

[mtorpey]

Oh yes, I missed the words "and a version".

Agreed, if the package request is already satisfied then it should not even be necessary to consult the package distro, and no download should be performed.