Access token err msg

[pururval]

This PR fixes two issues-

1. Improve Access Token Error message of src-cli #716

Improved the error message in login.go to emphasize that the access token is created by going to the webpage and the env vars are updated in the terminal.

3. Failing to build from source - dependency repo looks to be either private or deleted. #1114

As https://github.com/sourcegraph/sourcegraph was changed to https://github.com/sourcegraph/sourcegraph-public-snapshot
The dependency github.com/sourcegraph/sourcegraph/lib was not found. To fix this, a replace directive was added in the go.mod to update the path of that dependency to the new one.

Test plan

1. Tested by running go run ./cmd/src login to get the following as the expected result-

❌ Problem: No access token is configured.

🛠  To fix: Create an access token by going to https://sourcegraph.com/user/settings/tokens, then set the following environment variables in your terminal:

   export SRC_ENDPOINT=https://sourcegraph.com
   export SRC_ACCESS_TOKEN=(your access token)

   To verify that it's working, run this command again.

exit status 1

2. Unit tests

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestLogin$ github.com/sourcegraph/src-cli/cmd/src

=== RUN   TestLogin
=== RUN   TestLogin/different_endpoint_in_config_vs._arg
--- PASS: TestLogin/different_endpoint_in_config_vs._arg (0.00s)
=== RUN   TestLogin/no_access_token
--- PASS: TestLogin/no_access_token (0.00s)
=== RUN   TestLogin/warning_when_using_config_file
--- PASS: TestLogin/warning_when_using_config_file (0.00s)
=== RUN   TestLogin/invalid_access_token
--- PASS: TestLogin/invalid_access_token (0.01s)
=== RUN   TestLogin/valid
--- PASS: TestLogin/valid (0.00s)
--- PASS: TestLogin (0.01s)
PASS
ok      github.com/sourcegraph/src-cli/cmd/src  0.066s

[burmudar]

Thanks @pururval!