Feature Idea: Allow `pasta`ing only subset of dependencies

[Nearoo]

If a project has dependencies to multiple different places that might independently update, it might not be desirable to always copy all dependencies whenever pasta is executed.

Idea: Let users have multiple yaml files that list dependencies in a dedicated directory called .pasta. The names of each yaml servers as an identifier for the dependencies inside, and can be copied with pasta <name>.

Example directory structure

.pasta/
    foo.yaml
    bar.yaml
    baz.yaml

example commands:

pasta
pasta foo
pasta foo bar

Behaviour:

• Running pasta foo will copy all dependencies in foo.yaml, and generat generate foo.result.yaml placed inside .pasta
• Dependencies are read in alphabetical order, overwriting files will result in an error
• Running just pasta will execute all dependencies
• Users choose between having one top-level pasta.yaml, or multiple files in a .pasta/ directory; having both results in an error. Otherwise, it's not clear how only pasta.yaml (and no deps in the directory) should be executed

More commands:
pasta -l could list all dependencies. If dependencies are in a .pasta directory, a field description at top level can add a short description of what the dependencies in a file copy. Example output:

> pasta -l
Only toplevel pasta.yaml found; copy using "pasta"

> pasta -l
Found depencencies:
    proto - .proto files from repo xzy
    textures - texture images from repo abc