Support pack of global tools (PackAsTool)

[enricosada]

require .net sdk >= 2.1.300

using PackAsTool with Pack target, allow to create tools (for .net core are FDD)

this PR:

• remove the deps from generated nuspec
• set the deps assemblies so are added to nupkg

if is just a console app (no p2p, deps are ok).

dotnet new console -lang f#
dotnet pack /p:PackAsTool=true

if you want, just put the nupkg in a reachable feed (local is ok), and do dotnet tool install mytool -g (global) or dotnet tool install mytool --tool-path "%CD%\dir_with_tools" (to put tools in dir_with_tools, so dir_with_tools\mytool will work)

if is a console app with p2p, put the PackAsTool property inside the fsproj, otherwise the p2p deps are not added in the nupkg.

NOTE if we modify the condition to check the type (lib vs exe), will be possibile to allow also /p:PackAsTool=true with p2p, but i dont care atm

dotnet new console -lang f#
# put the PackAsTool property in fsproj
dotnet pack

Tests are disabled atm (but works), i'll upgrade everything to 2.1.300-rc1 with #3183 so can be run

[enricosada]

/cc @matthid if you need that for FAKE. i needed for #3183

[enricosada]

@forki @matthid ready to review for merge!

[matthid]

Yes looks good! We just need to remember to enable the tests later on, maybe we can add a bullet-point in the linked PR.