MSTest.Sdk: do not use IsImplictlyDefined

[Evangelink]

Do not use the IsImplictlyDefined="true" attribute in the PackageReference element in the .targets files. Instead,
rely on the VersionOverride attribute to define the package version. This is because for big projects, teams are usually
slowly migrating to MSTest.Sdk so they need to keep defining MSTest (and platform) packages in their CPM file.

If we use 'IsImplicitlyDefined' attribute, the package will be "defined twice" which will lead to NU1009 warnings that
are most of the time updated as errors (warnAsError). We created a thread with MSBuild and NuGet teams and the only suggested
solution is for users to suppress the warning which is not ideal. Until a better solution is provided, we will use the
VersionOverride trick instead as it achieves a relatively close behavior while preventing the warning.

We could also consider having a property like MSTestSdkDisableIsImplicitlyDefinedAttribute that users can set to true to
disable the IsImplicitlyDefined attribute in the .targets files but we don't see any strong reason to do that at the moment.