Fix ignored paths used by binary tool

[ellahathaway]

Closes dotnet/source-build#4391

The binary tool was not detecting binaries in repos where a .git repo was not present, because all files were being ignored upon initialization of said git repo.

Furthermore, some repos have been including binaries in their .gitignore files. This results in those binaries going undetected by the tool. See https://github.com/dotnet/dotnet/blob/8f3d7c26c231d54ad230f427a1005278c815dde6/src/fsharp/.gitignore#L43 as an example.

This PR changes the ignored paths from relying on the .gitignore files to simply hardcoding paths to ignore.

[mthalman]

Why are we ignoring certain binaries anyway? If we're not going to honor the .gitignore files of the repos, then what service are we providing to the developer anyway by keeping some binaries in the repo but not others?

[ellahathaway]

Why are we ignoring certain binaries anyway? If we're not going to honor the .gitignore files of the repos, then what service are we providing to the developer anyway by keeping some binaries in the repo but not others?

My only thought is that we can't rely on the .gitignore files, but we should still ignore certain paths like the git files, artifacts, and downloaded PSB packages. If someone wants to run just the validation for example, it would be unfortunate if the tool removed their build artifacts.

[MichaelSimons]

Per an offline discussion w/@ella, we concluded the following:

1. The use case of binary detection/stripping is from a clean environment. If you run the tool in a built environment, it is alright to detect all the built assets.
2. You can't trust/depend on .gitignore files because they can contain checked in files and we can't depend on the tool running in context of git
3. It is nice to have one place with the exclusions. Having certain paths hard coded in the tool in addition to the allowed-binaries txt file makes understanding the behavior more difficult.
4. We need to exclude certain well known paths in order for the tool to work correctly from the prep script - .git, .dotnet, .packages, prereqs/packages