handle git overrides in npm versions

[joprice]

Femto fails to run when a dependency has a git url as its version, for instance one starting with git+https://github.com..., which causes the following parse error:

Unhandled exception. System.ArgumentException: Invalid version string: "git+https"
   at SemVer.PartialVersion..ctor(String input)
   at SemVer.Comparator..ctor(String input)
   at SemVer.Comparator.TryParse(String input)
   at SemVer.ComparatorSet..ctor(String spec)
   at SemVer.Range.<>c.<.ctor>b__2_0(String s)
   at System.Linq.Enumerable.SelectArrayIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at SemVer.Range..ctor(String rangeSpec, Boolean loose)
   at [email protected](KeyValuePair`2 pair) in /Users/zaid/projects/Femto/src/Program.fs:line 149
   at Microsoft.FSharp.Collections.Internal.IEnumerator.map@99.DoMoveNext(b& curr) in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 102
   at Microsoft.FSharp.Collections.Internal.IEnumerator.MapEnumerator`1.System.Collections.IEnumerator.MoveNext() in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 84
   at Microsoft.FSharp.Core.CompilerServices.ListCollector`1.AddMany(IEnumerable`1 values) in D:\a\_work\1\s\src\FSharp.Core\seqcore.fs:line 502
   at Program.findInstalledPackages(String packageFile, PackageManager packageManager) in /Users/zaid/projects/Femto/src/Program.fs:line 144
   at Program.runResolution(Boolean resolve, FSharpOption`1 packageFile, PackageManager packageManager, FSharpList`1 libraries) in /Users/zaid/projects/Femto/src/Program.fs:line 992
   at Program.runner(FemtoArgs args) in /Users/zaid/projects/Femto/src/Program.fs:line 1677
   at Program.main(String[] argv) in /Users/zaid/projects/Femto/src/Program.fs:line 1782```