Implicit targets and file-level deps

[benjyw]

We want to minimize the amount of BUILD file boilerplate that is necessary to adopt Pants. In cases where we can figure out what the right thing to do is, we should do that even without corresponding BUILD file structures.

A big part of this involves inferring dependencies where possible, e.g., by analyzing import statements in .py files.
These inferred dependencies naturally exist at the file level (that is, they express a dependency of a single file on another single file). So it makes sense to be able to model dependencies at that level, instead of/as well as at the target level.

Having file-level deps allows us to have "implicit targets" at the file level. So, for example, if the user enters ./pants binary path/to/main.py we can act as-if there's a corresponding python_binary target, even if there isn't one.

[Eric-Arellano]

This is now implemented, outside of documentation, possible bug fixes, and the optimization of #10354.