Add support for xtest packages in Go Packages Driver

[JamyDev]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

xtests, also named external tests, are test cases that validate the public facing API of a package, even though the files are still located in the same package.

A sample folder structure:

code.go -> package code -> has both public and private methods/interfaces of things
code_test.go -> package code -> tests the private logic
code_ext_test.go -> package code_test -> tests the public API

From what I gather, the go compiler is happy to handle this since the package names are different, but when the Go packages driver runs its aspect, we'll get all of this in one FlatPackage (since they're part of the go_test rule) with the same package ID. This causes Go internally to mark the package code_test as inconsistent with the package code of the other test files.

The solution:

• Read the package id field from any file that has an _test.go suffix, if it doesn't match the package id provided by the FlatPackage, separate it out into a list of external test files.
• If we see that we have more than 0 xtestfiles, we create a new xtest package to add to the registry.
• Finally, we need to ensure that this xtest package gets added to the roots if we created one. We do this unconditionally, since if you're trying to resolve the tests of a package, you'd likely also want the external tests.