Support Go1.11 module

[tiancaiamao]

Feature Request

Go 1.11 introduce module, and it's the standard way for package dependency management.
Support module in TiDB may helpful, for example, #7884 can be fixed.
We'll also benefit from Go module that people will be able to use go get for the TiDB binary.

However, it's not a trivial work to support module.
The biggest problem is parser, as I've point out in #7494
parser.go is generated using make parser, this can't be handled by go module.

We can put generated parser.go in the repository directly, rather than generate it using Makefile script.
The drawback of this way is that every time parser.y is touched, there will be many lines change in parser.go. Then the TiDB repo will be inflate quickly.

A better way is moving parser to a separated repo, so every time parser is changed, only go.mod need to change accordingly.

[tiancaiamao]

I find gofail can't work properly with module