Check if references file exists on disk before installing into project file

Check if paket.references file exists on disk before allowing paket to blindly install/modify (csproj) project files with <Import>. If paket.references is missing from a project directory, paket should not touch any project files.
Also, according to @forki:

If you remove the paket.references file from the dotnetcore project folder
then it should ignore the csproj.

This is currently not true as there is no check here to exclude project files (csprojs) from the FindAllProjects enumeration. Additionally, this particular issue has caused some drama in issue https://github.com/fsprojects/Paket/issues/2029.
This PR resolves the issue and makes sure that paket.references exists on disk before paket does anything to MSBuild project files.
Also resolves #2220 by providing a way to opt-out of the <Import> mechanism.
Thanks,
Brian
:chocolate_bar: :cookie: :lollipop: Ronald Jenkees - Stay Crunchy

And, in addition, got your Travis CI build working again. :+1:
:walking: :sunglasses: "So don't delay... act now supplies are running out..."

@agross do you remember how we defined that case?
If a csproj doesn't have a corresponding references file - should treat it as empy or do we ignore the project?

@forki If a ??proj doesn't have a paket.references or the specific proj.??proj.references (not sure about the naming convention right now) file next to it, how would paket decide what to install in the first place. So I think yes, we need to ignore it.

how would paket decide what to install in the first place

currently it's creating an empty references file in memory. So it does not install anything - but it touches it since it wants to clean.

So it does not install anything - but it touches it since it wants to clean.

Yes it does install something, this right here:
<Import Project="..\.paket\Paket.Restore.targets" />

Even when there is no paket.references.   This  is  the   problem  in  #2029  and #2220 :exclamation::exclamation::exclamation:

I wished you would not see there targets as a problem. It's the only solution to get netcore working. They might not work for you yet. but it's the important part that needs fixing.