[BACKPORT] Allow Task.Source/Task.Sources to be constructed directly from subpath string literals

[lihaoyi]

First step in #4447, by providing an alternative to the previous os.Path APIs.

Effectively this allows us to replace

def mainScript = Task.Source { millSourcePath / "src/foo.py" }

with

def mainScript = Task.Source { "src/foo.py" }

Pulls in com-lihaoyi/os-lib#353 from upstream to make constructing os.SubPaths more ergonomic by eliding the lead os.sub / prefix in the case of literal strings while still maintaining a degree of safety:

• "outer" paths starting with ..s and absolute paths starting with / are rejected at compile time
• Only literal strings are converted implicitly, anything non-literal needs to be an explicit os.SubPath

For now we provide this as an alternative to passing in an absolute os.Path, but probably 99% of scenarios should be using this sub-path API rather than absolute paths since (a) it's more concise and (b) your sources should be within your millSourcePath anyway. I'm not sure we can get rid of the os.Path-taking API entirely, but we can definitely de-prioritize it and call it SourcesUnsafe or something so that anyone who needs it can use it but most people won't use it accidentally