Static analysis of `exportPattern()` directives

[lionel-]

Follow-up to #1325 and posit-dev/ark#870.

We now examine the DESCRIPTION and NAMESPACE files of a package to statically determine its exports, but we don't support the exportPattern() NAMESPACE directive yet, which exports all symbols matching a regex.

It's tricky because packages are installed without their source code, which means we can't statically discover what objects are defined in a namespace. To do this, we'll need access to the source code of the project dependencies: #2286. Then we'll be able to:

• Parse the source code and detect all top-level assignments

• For dynamic assignments, packages will have to declare the symbols:

  # Declaration
  foo <- function(my, signature) {}
  
  .onLoad <- function(...) {
    # Dynamic definition. This case is analysable in principle but the namespace
    # might be modified in more complex ways, e.g. via `.Call()`, hence the need
    # for declarations in general
    foo <<- function(my, signature) body
  }

• Apply export pattern to known namespace objects