bug(basenc): when appending a/ to a regular file regardless of the encoding, reports wrong error messages

Hi, uutils mainteners

i want let you known, that appending / to a regular file causes reporting wrong error messages no such file or directory, as our testing confirms
all encoding options have the same issue

relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base64url a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base32 a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base64 a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base32hex a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base16 a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base2lsbf a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base2msbf a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --z85 a/
basenc: a/: No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ ./basenc --base58 a/
basenc: a/: No such file or directory

while the GNU one correctly errors out

relunsec@relunsec:~/software/coreutils/target/debug$ gnubasenc --base64 a/
gnubasenc: a/: Not a directory
relunsec@relunsec:~/software/coreutils/target/debug$ 

Start by reproducing the listed basenc commands with a regular file followed by a/ and compare their diagnostics with GNU basenc. Trace basenc's input-path handling and error reporting across the encoding options; done means reporting “Not a directory” rather than “No such file or directory” for this case.