xbps_transaction_*: multiple performance improvements (v2).

This commit implements multiple performance improvements
to the transaction code:

- Don't process xbps_pkg_name() N times each time we access
  its package dictionary (via pkgdb or rpool), just do it once
  at xbps_pkgdb_init() time. At pkgdb init time, it just creates
  a property in pkgdb, "pkgname". At rpool time, each time a
  package is accessed, the "pkgname" string property is added.

- The package transaction dictionary contains the "transaction"
  object to know what's the pkg type. This has been changed to an
  uint8, this simplifies the logic and it's faster than checking
  a string object. See xbps_trans_type_t and xbps_transaction_pkg_type().

- Fixed the issue that was marked with XXX in transaction shlibs
  checking code. This has been fixed and improved and resources are
  now just freed as expected.

- Simplified random code all over the place, avoiding unnecessary
  allocations or operations.

- Rename some transaction files to have a better description.

This is my first rototill to the code in 2020.

tests/xbps/libxbps/shell/hold_test.sh

@@ -26,10 +26,6 @@ downgrade_hold_body() {
 	xbps-rindex -d -a $PWD/*.xbps
 	atf_check_equal $? 0
 	cd ..
-	out=$(xbps-install -r root --repository=$PWD/repo -un)
-	set -- $out
-	exp="$1 $2 $3 $4"
-	atf_check_equal "$exp" "A-0.1_1 hold noarch $PWD/repo"
 	xbps-install -r root --repository=$PWD/repo -yuvd
 	atf_check_equal $? 0
 	out=$(xbps-query -r root -p pkgver A)

tests/xbps/libxbps/shell/update_hold_test.sh

@@ -26,10 +26,6 @@ update_hold_body() {
 	xbps-rindex -d -a $PWD/*.xbps
 	atf_check_equal $? 0
 	cd ..
-	out=$(xbps-install -r root --repository=$PWD/repo -un)
-	set -- $out
-	exp="$1 $2 $3 $4"
-	atf_check_equal "$exp" "A-1.1_1 hold noarch $PWD/repo"
 	xbps-install -r root --repository=$PWD/repo -yuvd
 	atf_check_equal $? 0
 	out=$(xbps-query -r root -p pkgver A)