fix: go mod vendor

[cmgsj]

Fixes #61

• Vendoring with go mod vendor leads to "fatal error: 'pg_query.h' file not found"  #61

Inspired by:

• fix: go mod vendor goccy/go-graphviz#37

Changes:

• Generate gokeep.go files to (like .gitkeep) to force Go tooling to keep non-Go (.c and .h) files when vendoring.
• Generate parser/build_cgo.go file importing all gokeep packages.
• Make the above steps reproducible by running scripts/gokeep.sh on make update_source.

[lfittl]

Thanks for the contribution! - I'm definitely open to making this work, so we can drop the extra effort with vendoring the library.

Make update_source reproducible using git patch gokeep.patch.

I don't think that's the right approach, since Postgres major releases will bring different include folders, and so we'd have to update that patch each time.

What if instead we came up with a list of all include directories (through bash scripting) and then auto-generated the relevant keep files during the extract source step?

We could have each of the nested files use this template:

//go:build required
// +build required

// package gokeep prevents go tooling from stripping the c dependencies.
package gokeep

And then only have the list of imports at the top-level build_cgo.go file (which would take a string substitution of the list of nested files to be imported).

Do you want to adjust the PR to work like that?

[cmgsj]

//go:build required
// +build required

// package gokeep prevents go tooling from stripping the c dependencies.
package gokeep

Yeah, that makes sense, I'll update my PR. Thanks!