How to tell whether character string names a function that can be called in R, then call it?

Question

Suppose I have a character vector that contains the name of a function, such as basename, or stringr::fixed. I would like to:

1. Determine whether that function can be called in the current environment, and
2. Call the function using a list of arguments as with do.call.

My thought was this:

fn_chr = 'basename'
f = get0(fn_chr)
if(is.null(f) | !is.function(f)) stop('Function does not exist.')
args = list(pattern = 'w')
do.call(f, args)

[ignore the fact that pattern is not a valid argument for basename]

But this does not work for, say, stringr::fixed -

fn_chr = 'stringr::fixed'
f = get0(fn_chr)
if(is.null(f) | !is.function(f)) stop('Function does not exist.')
args = list(pattern = 'w')
do.call(f, args)

get0 does not find the function, and do.call, even if given the string, won't find it either.

do.call(fn_chr, args)
# Error in `stringr::fixed`(pattern = "w") : 
#   could not find function "stringr::fixed"

But I have stringr installed, so

stringr::fixed(pattern = "w")

works fine.

How can I construct the function call that I want?