bug#33741: Packages built with Go 1.10+ keep unnecessary references to Go inputs

[Pierre Neidhardt <mail@ambrevar.xyz>]

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See https://lists.gnu.org/archive/html/guix-devel/2018-11/msg00223.html.

Go binaries are statically build, but since Go 1.10, the full paths to
the Go libraries are kept in the binaries, which results in the Go
dependencies being part of the package closure:

$ guix gc --references $(./pre-inst-env guix build --no-grafts kurly)
/gnu/store/2b2md66fbzyspsmd5dj6zkj9hilac40r-tzdata-2018e
/gnu/store/4iwksvq53rlzphfp3xvp63ihlw226c0n-go-github-com-aki237-nscjar-0.0.0-0.e2df936
/gnu/store/5rxdjbk8h0bh1hbaan8y8ib13va2bcmw-net-base-5.3
/gnu/store/ahvdlp6y44qj6kx63rmx1sq8r61x3zc2-go-github-com-alsm-ioprogress-0.0.0-0.063c372
/gnu/store/f8yps0l8p371jgzh6cki0z5n2kgfjiwy-go-github-com-urfave-cli-1.19.1-0.934abfb
/gnu/store/l4lr0f5cjd0nbsaaf8b5dmcw1a1yypr3-glibc-2.27
/gnu/store/pp0bakrbyv9xmp1kyv2114l19s11b74z-gcc-6.4.0-lib

Previously, they did not:

$ guix gc --references $(guix build --no-grafts kurly)
/gnu/store/2b2md66fbzyspsmd5dj6zkj9hilac40r-tzdata-2018e
/gnu/store/5rxdjbk8h0bh1hbaan8y8ib13va2bcmw-net-base-5.3
/gnu/store/l4lr0f5cjd0nbsaaf8b5dmcw1a1yypr3-glibc-2.27
/gnu/store/pp0bakrbyv9xmp1kyv2114l19s11b74z-gcc-6.4.0-lib

It seems to be an upstream bug:

https://github.com/golang/go/issues/16860

It's still unresolved and only planned for Go 1.13.

Note that adding

              "-asmflags=all=-trimpath=/gnu/store"
              "-gcflags=all=-trimpath=/gnu/store"

to the build system does not work, because we need to trim the hash too.
-trimpath supports only one parameter, so we can't use it for this purpose.

We could use Boyer-Moore to replace the hashes of all Go dependencies.
We must be careful to only replace paths to Go libraries.

--
Pierre Neidhardt
https://ambrevar.xyz/

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