internal/mod/modload: preserve custom struct in module when tidying requirements

This updates modfileFromRequirements to copy the custom struct from the
original modfile to the tidied one.

Fixes #3876

Closes #3877 as merged as of commit 4398218.

Signed-off-by: Stefan Nuxoll <[email protected]>
Change-Id: I3e1b57aeb9570aabc0da992f491a445998b80fcf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1212931
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: snuxoll

Diff for: internal/mod/modload/testdata/tidy/preserve-custom.txtar

@@ -0,0 +1,20 @@
+-- cue.mod/module.cue --
+module: "cue.example"
+language: {
+	version: "v0.12.0"
+}
+custom: {
+	"example.com/mycoolmodule": {
+		test: "data"
+	}
+}
+-- want --
+module: "cue.example"
+language: {
+	version: "v0.12.0"
+}
+custom: {
+	"example.com/mycoolmodule": {
+		test: "data"
+	}
+}

Diff for: internal/mod/modload/tidy.go

@@ -156,6 +156,7 @@ func modfileFromRequirements(old *modfile.File, rs *modrequirements.Requirements
 		Language: old.Language,
 		Deps:     make(map[string]*modfile.Dep),
 		Source:   old.Source,
+		Custom:   old.Custom,
 	}
 	defaults := rs.DefaultMajorVersions()
 	for _, v := range rs.RootModules() {