install.zsh: allow alternatives in deps

Specify a dependency as a|b to check if either a or b is available as a
command. The script itself must check at runtime, which one is actually
available and use the right one. Helper methods for this are provided in
lib/common.zsh:
  - `alternatives a b` aliases a to b, if a is not available
  - `alternatives-noparam a b` replaces a with b, if a is not available,
    and discards any parameters (e.g. for replacing a formatter with cat)

install.zsh

@@ -6,14 +6,15 @@ typeset -A interpreter_checked
 have_dependency() {
 	if [[ ! $interpreter_checked[$1] ]]; then
 		check "Checking for $1… "
-		if which $1 &> /dev/null; then
-			succeed
-			interpreter_checked[$1]=1
-			return 0
-		else
-			fail "not found in path"
-			return 1
-		fi
+		for variant in ${(s.|.)1}; do
+			if which $variant &> /dev/null; then
+				succeed
+				interpreter_checked[$1]=1
+				return 0
+			fi
+		done
+		fail "not found in path"
+		return 1
 	fi
 
 }

lib/common.zsh

@@ -26,6 +26,14 @@ exists() {
 	type "$1" &>/dev/null
 }
 
+alternative() {
+	if ! exists $1; then eval "$1(){ $2 \$@}"; fi
+}
+
+alternative-noparam() {
+	if ! exists $1; then eval "$1(){ $2 }"; fi
+}
+
 depend() {
 	local missing
 	local i