Allow : in target and filenames if not the first character

[fmeum]

Especially in the Perl community, filenames containing : are common. Labels can still be parsed unambiguously if target names can contain colons by declaring that the first colon in a given label string is interpreted as the separator between package and target name. This only requires prefixing with : when referring to targets containing colons in the same package.

Target and filenames starting with : are still not allowed to prevent confusion with references to targets.

Work towards #374

RELNOTES: Target and filenames can now contain : as long as it is not the first character. Same-package references to such targets have to be prefixed with :.

[fmeum]

@meisterT I'm trying to make some progress on #374 and #4327. What do you think of the approach taken by this PR? Space and colon appear to be the most commonly used characters unsupported by Bazel and it looks like supporting colon wouldn't be too problematic with a change like this.

[Wyverald]

There was a lengthy internal debate about how to handle such things. @brandjon

[fmeum]

There are more tests that expect to fail when encountering a :. I will fix those up if the PR is otherwise viewed favorably.

[brandjon]

Thanks for the PR.

We had a lot of internal discussion about admitting more label characters, and came up with a design that we didn't prioritize implementation of yet. I've paged it out of my head by now, but a key feature is that it adds an escaping scheme rather than allowing raw colons in the target part of the name. This is important to avoid breaking existing code (both in Starlark and in tooling that runs outside of the build itself) that expects to be able to parse labels based on naive matching of the colon character.

If we prioritize this capability, I'd be happy to discuss the design in more detail, but we don't have the bandwidth at the moment. In the meantime, it would be dangerous for us to allow raw colons in labels, and also dangerous to extend label syntax in other ways we haven't deeply thought through, since it's hard to reclaim pieces of the namespace we've given away.