build-system/go: Build with a filesystem union of Go dependencies.

This basically changes (guix build-system go) so that instead of looking
up its dependencies in a list of directories in $GOPATH, all the
Go dependencies are symlinked into a single directory.

Fixes <https://bugs.gnu.org/33620>.

* guix/build/go-build-system.scm (setup-go-environment): New variable.
(setup-environment, install-source): Remove variables.
(unpack): Unpack the source relative to $GOPATH.
(install): Do not install the compiled objects in the 'pkg' directory.
Install the source code in this phase, and only install the source of
the package named by IMPORT-PATH.
* doc/guix.texi (Build Systems): Adjust accordingly.
* gnu/packages/docker.scm (docker): Import (guix build union) on the build side
and adjust to build phase name changes in (guix build-system go).
* gnu/packages/shellutils.scm (direnv): Likewise.
* gnu/packages/databases.scm (mongo-tools)[arguments]:
Set '#:install-source #f'.
* gnu/packages/music.scm (demlo)[arguments]: Move the 'install-scripts'
phase after the 'install' phase.

gnu/packages/docker.scm

@@ -249,9 +249,11 @@ network attachments.")
      `(#:modules
        ((guix build gnu-build-system)
         ((guix build go-build-system) #:prefix go:)
+        (guix build union)
         (guix build utils))
        #:imported-modules
        (,@%gnu-build-system-modules
+        (guix build union)
         (guix build go-build-system))
        #:phases
        (modify-phases %standard-phases
@@ -412,8 +414,8 @@ network attachments.")
              ;; Make build faster
              (setenv "GOCACHE" "/tmp")
              #t))
-         (add-before 'build 'setup-environment
-           (assoc-ref go:%standard-phases 'setup-environment))
+         (add-before 'build 'setup-go-environment
+           (assoc-ref go:%standard-phases 'setup-go-environment))
          (replace 'build
            (lambda _
              ;; Our LD doesn't like the statically linked relocatable things