Fix PackageSaveMode nuspec always reinstalls

[huangqinjin]

Bug

Fixes: NuGet/Home#2402

Regression? Last working version: No

Description

Check whether a package with same PackageSaveMode is already installed. That would fix the linked bug.
Enable PackageSaveMode related unit tests.

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
  • OR
  • N/A

• Documentation

  • Documentation PR or issue filled
  • OR
  • N/A

[dnfgituser]

All CLA requirements met.

[erdembayar]

@huangqinjin
Thank you for your contribution. Could you be able to fill description part of your PR?
Also please add unit tests here for your change.

[huangqinjin]

@erdembayar done.

[erdembayar]

@erdembayar done.

LGTM. But just in case let's wait others to review it, because I'm not familiar this part of code so there could be edge cases.

[erdembayar]

@NuGet/nuget-client
Please review.

[erdembayar]

@nkolev92
Could be able to take a look here?
LGTM but still not confident to approve. I don't have much experience with PackageSaveMode.

[erdembayar]

@huangqinjin
Thank you for your approval. I'll merge today evening or tomorrow.