For greppability, those files are:
    FSharpSource.targets
    FSharpSource.Settings.targets

I'm doing this because I had trouble remembering which file gets included
at which end of a project file, which is pretty important to know when
thinking about whether you can use such-and-such property in either a
project file or the other .targets file, or which file a given property
definition needs to appear in.

Since I plan to do away with the -proto proto projects in favor of the
unsuffixed projects built with Configuration=Proto, it seems wise to
add validation that the project in question actually supports this.

This only affects references to non-F#-related libraries.

The point of this commit is to verify that there's no particular
reason for these references to be different in proto builds.  If
there is, then the resulting "git bisect" should finger this commit.

Visual Studio already knows about these files from their CustomCopyLocal
items, so having Content items for them too can only serve to confuse.

This was mostly done by copying and pasting the items from the normal
projects to the -proto projects, then commenting some .fs items out in
FSharp.Compilers-proto.fsproj (at least one of which broke the build until
I commented it out).  (Also fixing up a few relative paths, naturally.)

*Except it actually doesn't do the proto build at the moment.**

**Or the VS integration.

We didn't need that half, honest!

Here we get to replace that huge batch of msbuild invocations not just
once, but twice, since it appeared in both Debug and Release variants.