Muttrc: how to source a file in muttrc's directory - relative-path

Question

I have a muttrc file which sources a secondary file mutt-secrets which resides in its same directory. But I have what appear to be two conflicting needs:
Be free to reference the muttrc file from any working directory
Be free to move it (and mutt-secrets) without having to edit muttrc to change the source path for mutt-secrets
At present, the first line of my muttrc says: source mutt-secrets. That works fine when I run mutt from within the directory where the two file reside, but if I run mutt from elsewhere and reference muttrc with a -F flag, then mutt can find muttrc, but muttrc can't find mutt-secrets.
How can I solve this?

Answer 1

Use absolute paths. For example:
source ~/.mutt/mutt-secrets

Answer 2

TL;DR one-line solution:
source `lsof -c mutt -Fn | grep '/muttrc$' | sed 's|^n||; s|/muttrc$||;'`/mutt-secrets
or, if you want to reuse the muttrc directory, you can save it to a custom variable:
set my_muttrc_dir = `lsof -c mutt -Fn | grep '/muttrc$' | sed 's|^n||; s|/muttrc$||'`
source $my_muttrc_dir/mutt-secrets
If you want to see the output of the command when you launch mutt, you can put this line in your muttrc:
echo `lsof -c mutt -Fn | grep '/muttrc$' | sed 's|^n||; s|/muttrc$||'`
Assumptions: the Mutt process is called mutt and the Mutt's initialization file is called muttrc. Furthermore, you could get in trouble if you have more than one Mutt instance running (for example if you launch in parallel two or more Mutt instances with different initialization files, because the command may select the wrong path).
Explanation
The idea is to look for muttrc full path in the list of open files by Mutt. We can get this list using lsof command (it has to be installed in your system), then extract the full path by parsing the lsof output with grep and sed commands.
This approach is viable because Mutt's initialization files support the use of external command's output with backticks (``). When Mutt encounter and execute our command enclosed in backticks (``), it is in the process of reading the muttrc file, so the muttrc file appears in the list of currently open files by Mutt. This enables us to use the lsof command.
lsof parameters
-c mutt: list open files of process named mutt;
-Fn: for each element, print only the name (it is the path in our case). Because of lsof output format, the path will be prefixed with the character n.
grep and sed
We use grep to select the line which contains muttrc file path, assuming the filename is exactly muttrc. Then we clean the lsof output with sed by both removing the n character at the beginning of the line and the /muttrc string from the end of the line. This way we get the path of the directory containing the muttrc file.
There is a cleaner solution?
Mutt expands relative paths inside initialization files from its current working directory, i.e. from the directory you launch Mutt. It supports a mechanism that allows path's expansion relatives to something different, but the "initialization file directory" or something similar are not available. See here.
I neither found a way to get the -F <path> option you pass to the mutt command inside the initialization file.
References
backticks in Mutt's initialization file;
current directory;
_mutt_buffer_expand_path, source code
source_rc, source code
source_rc call, source code
Tested with: Mutt 2.0.5, lsof 4.93.2, GNU grep 3.7, GNU sed 4.7.