Add repo dialog will accept repo names which don't contain a /

[dominiccooney]

This speculatively asks the remote to match the raw user input and the user input with the clone URL to repo name heuristic applied to it.

VSCode does not have this issue because repos are picked from a list retrieved from the remote, instead of input manually.

Fixes #1194

Test plan

Tested manually:

• Breaking in validateRepoExists and check that we do fetch repo IDs for repo names without /, eg foo
• Check remotes like ssh@github.com:sourcegraph/jetbrains still work
• Check duplicate checks are still insensitive to exact input syntax, eg ssh@github.com:sourcegraph/jetbrains is detected as a duplicate of github:sourcegraph/jetbrains and https://github.com/sourcegraph/jetbrains.git, etc.

[pkukielka]

LGTM