fix bug in i18n merge_file/itstool_join revealed by previous commit

Logically, i18n.merge_file cannot ever take a MULTI_OUTPUT_KW, but it
does take a CT_OUTPUT_KW-like interface.

Actually trying to pass multiple merge_file outputs causes the
msgfmthelper script to be entirely malformed in the arguments it
accepts, and treat the broken one like a --flag, then exit with argparse
errors.

Even if we somehow assumed that somehow it was designed to actually
allow this, msgfmt doesn't support conceptually passing multiple outputs
so that would be a msgfmt error instead of an error inside the guts of
`meson --internal msgfmthelper`.

Same logic applies again for the itstool command and the itstool
internal helper.

Catch this error at configuration time by using the single-output kwarg
form.

Likewise, it's totally nonsense to accept multiple install_dir or
install_tags, and ever since commit 11f96380351a88059ec55f1070fdebc1b1033117
the CustomTarget itself won't even check this.

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