Make readDir/readFile more precise and flexible

[bep]

See #9599

There are historical reasons to this, I'm sure, but the current behaviour is hard to understand, document and is not as flexible as it easily could be.

So, I suggest adding a filesystem option to both of these functions:

• work -- default, will search in the project work dir only
• content (composite, project first)
• archetypes (composite, project first)
• static (composite, project first)
• layouts (composite, project first)
• data (composite, project first)
• assets (composite, project first)
• i18n (composite, project first)

The new default (that work is project work dir only) is a slightly breaking change, but I suspect it's going to solve more issues than it creates.

[bep]

I have marked this as a "GoodFirstIssue" which I think is true, as it's very isolated. Add some failing testcases to tpl/os/integration_test.go would be a good start. I suggest adding a GetFilesystemByName or something to the BaseFS struct and use the constants infiles.