Speed up check_installed() - shortcut when package is already loaded

[shikokuchuo]

Can safely shortcut when package versions are not specified.

This prevents the full code path being run every time the function is called within a session, and is equivalent to the user memoizing the result.

Takes 3 microseconds, which is not much to give back on the initial run when we've saved some hundreds of microseconds in #1777.

library(rlang)
bench::mark(check_installed("later"))
#> # A tibble: 1 × 6
#>   expression                        min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>                   <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 "check_installed(\"later\")"   2.47µs   3.01µs   305568.    5.61MB     30.6

^{Created on 2025-02-20 with reprex v2.1.1}

This should be applied with #1777 but is independent of that.
Closes #1776.

[lionel-]

Can you please add a comment about what happens when a version is supplied?

[shikokuchuo]

Added in c65352f.