net45 support for nuget.packaging.extraction.dll and transitive closure

[rrelyea]

Fixes: NuGet/Home#9883 - covers motivation, etc..
Regression: No

Last working version: N/A
How are we preventing it in future: N/A

Fix
Details: see at bottom of this comment.

Testing/Validation
Tests Added: No
Reason for not adding tests: Deadline...same code base. Tracking issue to ensure proper coverage: NuGet/Home#9884

Validation: Successful CI run, VS Setup team running bits and verifying rejection of tampered packages, etc...

Details of fixes:

• worked around net45 localization problem in common.targets, bootstrap.proj and buildvalidator.ps1 -- follow up for proper fix: https://github.com/NuGet/Client.Engineering/issues/459

strategy for fixes.

• use #if !net45 and #else as necessary to provide alternate techniques that work on net45.
• built ireadonlycollectionutility to abstract HashSet not supporting IReadOnlyCollection in net4.5
• built marshalutility to abstract generic way, vs earlier way
• ifdef for empty array creation. new way has perf optimizations to share instances.
• compiler made me fix a few things: -> nameof(foo), count() -> any()
• signing functionality -- use alternate techique to get RSAPublicKey - @dtivel please review
• dispose wasn't supported in 4.5 on a few types (x509 related) - skipping dispose best solution - @dtivel please review

new dll - nuget.packaging.extraction.csproj - mostly linked to the subset of code files from nuget.packaging that needed to pull in for compilation. pulled over copies of resx files. didn't subset those resx files.

[dominoFire]

Some source-build comments.

[aortiz-msft]

[nkolev92]

I think we can minimize the impact by reducing the number of projects that build net5 assemblies.

Might be easier to compile everything into 1 assembly.

Ideally we don't publish this on NuGet.org, that will require changing our release definition.

[nkolev92]

@rrelyea Given that my feedback is summarized in https://github.com/NuGet/Client.Engineering/issues/461, I see no reason to wait on me here.

[dtivel]

Optimistically approving. Request a rereview from me if there are non-trivial changes.

@rrelyea, since X509Certificate didn't implement IDisposable until .NET 4.6, your changes look appropriate.