Migrate to Go modules

Currently, we use dep in most of our PMM repositories, but it is not supported. We should migrate to Go modules while keeping our build and release processes working all the time.

For this ticket, let's keep vendor/ directory as checking its diff will be a helpful way to check that migration actually worked.

We also should remove all dependencies that are not added back by go mod tidy.

Proposed plan:

1. Migrate pmm-api-tests – repository without tags and releases.
2. Update exporters to use upstream's build system with modules (with or without vendoring depending on upstream) without updating / merging with their source code.
3. Migrate pmm-admin and pmm-agent – repositories that we build and release but without PMM-2.0 branches or v1 tags.
4. Migrate pmm-update – repository with PMM-2.0 branch.
5. Migrate qan-api2 and pmm-managed.
6. Migrate pmm repo – the hardest, as it is an incompatible v2 module that is used by other repositories.
7. Update all repositories to use pmm module – that will remove many dependencies from them.

Community contributions:

• https://github.com/percona/pmm-managed/pull/524
• https://github.com/percona/pmm-admin/pull/127
• https://github.com/percona/proxysql_exporter/pull/42