docs: package to be linted should be installed

[dpprdan]

Linting a package that is not installed may generate false positives for (at least) object_usage_linter, see e.g. r-lib/actions#557

1. Why is that exactly? Is object_usage_linter (and probably others) use namespace of installed package #91 still relevant here?
2. (Where) should this be documented?

Re 2: I took

lintr/README.md

Lines 217 to 218 in fff7808

	If you want to run `lintr` on [Travis-CI](https://travis-ci.org), you will need
	to have Travis install the package first. This can be done by adding the

to mean that the package to be linted must be installed, but I guess it just means that lintr needs to be installed (which is self-evident, though, I suppose)? Anyway, my first thought was to document it at the beginning of the the

lintr/README.md

Line 192 in fff7808

### For packages

section, because it is also valid for GHA.

But the "package should be installed" applies locally as well, so maybe it should be in the lint_package() docs? Maybe something like

Note: To avoid false positives, the package to be linted needs to be installed.

[AshesITR]

Not sure if useful info, but using devtools::load_all() should suffice, no actual installation required.

The problem is lintr only lints one file at a time so it can't see function definitions from around the codebase during linting.
As a work-around, the package can be loaded in order to make all symbols visible in the globalenv.

A similar issue will happen when linting regular files that source() other functions.

I'm actually confused why simply installing the linted package fixed the issue at all.

[MichaelChirico]

I'm thinking the way to document this is with tags -- it only really applies to executing linters, so we should strive to reflect our understanding in ?executing_linters:

lintr/R/linter_tag_docs.R

Lines 93 to 101 in fa914d0

	#' Code executing linters
	#' @name executing_linters
	#' @description
	#' Linters that evaluate parts of the linted code, such as loading referenced packages.
	#' These linters should not be used with untrusted code, and may need dependencies of the linted package or project to
	#' be available in order to function correctly.
	#' @evalRd rd_linters("executing")
	#' @seealso [linters] for a complete list of linters available in lintr.
	NULL

And maybe mention this class of linters in the README/intro vignette.