Fix transitive content conflicts Fixes #3871

[Forgind]

Changes this to opt-out for sdk-style projects. (See discussion in microsoft/msbuild#4931 and microsoft/msbuild#4997.)

Fixes #3871.

[dsplaisted]

This also fixes #3864, correct?

[sfoslund]

@dsplaisted I don't think so, this ensures that transitive content files don't overwrite other content files of the same name. If I'm reading #3864 correctly, that issue refers to copying all transitive content, regardless of name.

[sfoslund]

@Forgind @dsplaisted the corresponding MSBuild PR was merged, right? So this can be merged too?

[Forgind]

Can confirm:
dotnet/msbuild#4931

[sfoslund]

@Forgind cool, I'm merging with master to get the new version of MSBuild that has the change, just to get CI run with it enabled.

[rainersigwald]

Should this have a test and an inverse test for when it's explicitly disabled?

[sfoslund]

@rainersigwald sure, I just pushed the test I wrote back in November 😆. It's a little messy because this issue is hard to repro/ detect consistently.

[sfoslund]

Weird, this tests is reliable on my local machine but it looks like it's really flaky in general. @Forgind were you able to get a consistent repro when you were testing that could be put into this testing format?

[Forgind]

It seemed to repro reasonably consistently if I remember correctly, but I just manually tested it, and unfortunately, this isn't very high on my priority list right now. I can try to get back to it in another month or so.

[sfoslund]

I've added a new test that asserts the target output instead of the file that ends up in the output folder, which is unreliable when there are conflicting content files. This one should be much more reliable and doesn't require adding a new test asset.

@dsplaisted can you review, since I've added code?