Fork of godef is no longer needed for Go modules support

[ramya-rao-a]

Now that the godef fork https://github.com/ianthehat/godef has been merged back to the original repo, we no longer need to use it for Go modules support

• Remove godef-gomod from the list of tools to install/update in goInstallTools.ts file
• Add a notification to let the user know that they need to update their version of godef tool. This notification should be shown atmost once in a VS Code session.

[SylvainGarrigues]

Hey, godef-gomod doesn't work any longer in vscode with modules, but godef does.

Until https://github.com/Microsoft/vscode-go/blob/master/src/goDeclaration.ts#L83 is updated to remove reference to this tool, I found out that

# cd ${GOPATH}/bin && rm godef-gomod && ln -s godef godef-gomod

Makes vscode usable again with Go modules and definitions :)

[Delphier]

VSCode: 1.30.1
Go Extension: 0.8.0
Go 1.11.4
Enabled go.mod

godef-gomod install failed:

# cd ........\src\github.com\ianthehat\godef; git pull --ff-only
fatal: Not possible to fast-forward, aborting.
package github.com/ianthehat/godef: exit status 128

rename godef.exe to godef-gomod.exe, works fine.

[oneslash]

@ramya-rao-a, I am almost done with PR, however, I am not sure how to display one time notifications.

[ramya-rao-a]

@oneslash Do you have a PR open?

[oneslash]

@ramya-rao-a here is it #2234

[ramya-rao-a]

I have merged your PR. I'll take care of the notification in another PR that I am currently reviewing