Relocate agent

[edigaryev]

The first commit contains a lost of changes and can be reproduced by running the following commands:

git clone https://github.com/cirruslabs/cirrus-ci-agent.git agent
mkdir internal/agent
mv agent/cmd/agent/* agent/internal/* internal/agent/
mv agent/api agent/pkg/grpchelper pkg/
find cmd/ internal/ pkg/ -type f -print0 | xargs -0 sed -i '' -e 's|github.com/cirruslabs/cirrus-ci-agent/internal/|github.com/cirruslabs/cirrus-cli/internal/agent/|g'
find cmd/ internal/ pkg/ -type f -print0 | xargs -0 sed -i '' -e 's|github.com/cirruslabs/cirrus-ci-agent/api"|github.com/cirruslabs/cirrus-cli/pkg/api"|g'
find cmd/ internal/ pkg/ -type f -print0 | xargs -0 sed -i '' -e 's|github.com/cirruslabs/cirrus-ci-agent/pkg/grpchelper"|github.com/cirruslabs/cirrus-cli/pkg/grpchelper"|g'
rm -rf agent
go mod tidy
go fmt ./...

The second commit is small and only accounts for manual changes.

Unfortunately this also introduces a chicken-and-egg problem where we cannot fully stop depending on the cirruslabs/cirrus-ci-agent repository until we make a new release in this one, here's an example:

cirrus-cli/internal/executor/platform/windows.go

Lines 52 to 53 in 1f12770

	windowsAgentURL := fmt.Sprintf("https://github.com/cirruslabs/cirrus-ci-agent/releases/"+
	"download/v%s/agent-windows-amd64.exe", DefaultAgentVersion)

I also had to disable the golangci-lint for internal/agent/ directory because the CLI's linter settings are more strict.