Ensure that nuget dependency path exists before reading

[jonabc]

A corollary to #273 to check that the path to a nuget dependency exists before calling File.read. This is separated from the referenced PR as that is waiting on some feedback from the dotnet team and I'd like to put out a new release soon.

Besides making the ☝️ aforementioned fix, this also slightly changes the functionality of the dependency to be just a bit more performant when:

1. not using the cache command
2. using the cache command when no changes are needed to cached metadata

These make up the broad majority of licensed runs, so hopefully this makes things a little bit faster. Two specific changes here:

1. Don't get the project_url or description until evaluating a dependency's license_metadata. That function is only called when caching new metadata or overwriting existing metadata, and takes reading and parsing nuspect_path out of the common path.
2. Move checks on nuspec_path and nuspec_contents into each cached begin/end block. This has the effect of caching a nil value rather than not caching a value + re-porforming those checks when they evaluate to false.

/cc @paveliak @zarenner FYI on parceling out some of the fix from the referenced PR

[zarenner]

Nice! I wanted to do something like this but I think wasn't sure if it was cool to override license_metadata. Also TIL about tap, thanks for the knowledge 👍

[paveliak]

Thinking of it in terms on C# is it akin to overriding virtual property? That is:

1. Get property value from base class
2. Assign two metadata values
3. Return modified property

[jonabc]

@paveliak yup 👍 . In ruby all methods can be overridden by default. There is no special syntax like virtual and override which explicitly tell which methods can be overridden and which methods are overriding base classes. The call to super here is equivalent to calling base.licensed_metadata() in c#.