src/goInstallTools.ts: update install instructions for dlv-dap

Since dlv-dap is just dlv built at master and renamed, the standard
install instructions do not apply. Update to link to the documentation.

Also, look for dlv-dap tool instead of dlv when launching the dlv dap
process.

Updates golang/vscode-go#1395

Change-Id: I7639919594b1cbd753d09386794d0774f785122e
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/307054
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/src/goDebugFactory.ts b/src/goDebugFactory.ts
index 8005199..0e207a4 100644
--- a/src/goDebugFactory.ts
+++ b/src/goDebugFactory.ts
@@ -275,12 +275,12 @@
 	const launchArgsEnv = launchArgs.env || {};
 	const env = Object.assign({}, process.env, launchArgsEnv);
 
-	const dlvPath = launchArgs.dlvToolPath ?? getTool('dlv');
+	const dlvPath = launchArgs.dlvToolPath ?? getTool('dlv-dap');
 
 	if (!fs.existsSync(dlvPath)) {
 		const envPath = process.env['PATH'] || (process.platform === 'win32' ? process.env['Path'] : null);
 		logErr(
-			`Couldn't find dlv at the Go tools path, ${process.env['GOPATH']}${
+			`Couldn't find dlv-dap at the Go tools path, ${process.env['GOPATH']}${
 				env['GOPATH'] ? ', ' + env['GOPATH'] : ''
 			} or ${envPath}`
 		);
diff --git a/src/goInstallTools.ts b/src/goInstallTools.ts
index 287489e..4ea028e 100644
--- a/src/goInstallTools.ts
+++ b/src/goInstallTools.ts
@@ -354,8 +354,12 @@
 		// Offer the option to install all tools.
 		installOptions.push('Install All');
 	}
-	const msg = `The "${tool.name}" command is not available.
+	let msg = `The "${tool.name}" command is not available.
 Run "go get -v ${getImportPath(tool, goVersion)}" to install.`;
+	if (tool.name === 'dlv-dap') {
+		msg = `The "${tool.name}" command is not available.
+Installation instructions can be found at https://github.com/golang/vscode-go/blob/master/docs/dlv-dap.md#updating-dlv-dap.`;
+	}
 	const selected = await vscode.window.showErrorMessage(msg, ...installOptions);
 	switch (selected) {
 		case 'Install':