Does not require import paths on build tag'd files

[djui]

If a file (e.g. foo_test.go) includes //+build integration, it won't touch it to rewrite it.

[marwan-at-work]

@djui thanks for bringing this up.

I added a new flag so you can do mod upgrade -buildflags=-tags=integration

This is because the go compiler has no option to parse all go files so we have to be explicit about the build tags.

I'll think about whether it's worth replacing go/packages with manual parsing of go files so that we avoid having to explicitly pass -buildflags

I might also think about replacing -buildflags with just -tags=one,two,three to make the mod API a little bit nicer. Let me know what you think :)