Define dependency to scala files in using directives

[lwronski]

No description provided.

[alexarchambault]

There are a few questions I'm wondering about the new inputs:

• are they taken into account when watching files on disk? (IIRC, we only watch things from the initial Inputs instance)
• do they work fine from BSP? (they should, but I'd add a test for that in BspTestDefinitions just in case)
• are relative paths computed fine? like what if the using file directive is in a source in a sub-directory, like in src/foo/bar/Something.scala?

[Gedochao]

I think it might be cool to highlight this feature in the scripts.md guide as well, since that's where this feature would probably be at its most useful.

[lwronski]

There are a few questions I'm wondering about the new inputs:

• are they taken into account when watching files on disk? (IIRC, we only watch things from the initial Inputs instance)

Now it should work, and upon every change, I re-compute actual Inputs instance adding sources from using directives.

• do they work fine from BSP? (they should, but I'd add a test for that in BspTestDefinitions just in case)

Yes, It works fine from BSP and I also added one BSP test.

• are relative paths computed fine? like what if the using file directive is in a source in a sub-directory, like in src/foo/bar/Something.scala?

I think, it is out of the scope of this PR, it should be fixed in #1098. Now, it works the same as path resolving for the resourceDir parameter in using directive.

[alexarchambault]

Apart from the minor commit (and maybe reworking the commits a bit so that we can do a merge commit), LGTM!