bin/xbps-create: add support for recording provides priority information

[classabbyamp]

The provides-priority property is specified as a space-separated array of comma-separated keys and values that record the priority which xbps should give to a virtual package a package provides. Keys are pkgvers, and values are positive integers. Higher values will be given a higher priority, and missing entries will be given a priority of 0 by default.

This is implemented as a generic function with validation and parsing callbacks so it can be used for other properties that are dictionaries.

The alternatives property handling is genericised too.

example:

xbps-create -A noarch -n foo-1.0_1 -s "foo pkg" \
    -P "cmd:foo-1.0_1 foo-0_1 awk-0_1" \
    --provides-priority "foo-0_1,100 awk-0_1,25" \
    --alternatives "vi:vi:/usr/bin/nvim vi:vi.1:/usr/share/man/man1/nvim.1 vi:view:/usr/bin/nvim vi:view.1:/usr/share/man/man1/nvim.1 vim:vim:/usr/bin/nvim vim:vim.1:/usr/share/man/man1/nvim.1" \
    /tmp/pkg

creates this props.plist (only diff between this branch and master is the new property):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>alternatives</key>
  <dict>
    <key>vi</key>
    <array>
      <string>vi:/usr/bin/nvim</string>
      <string>vi.1:/usr/share/man/man1/nvim.1</string>
      <string>view:/usr/bin/nvim</string>
      <string>view.1:/usr/share/man/man1/nvim.1</string>
    </array>
    <key>vim</key>
    <array>
      <string>vim:/usr/bin/nvim</string>
      <string>vim.1:/usr/share/man/man1/nvim.1</string>
    </array>
  </dict>
  <key>architecture</key>
  <string>noarch</string>
  <key>installed_size</key>
  <integer>0</integer>
  <key>pkgname</key>
  <string>foo</string>
  <key>pkgver</key>
  <string>foo-1.0_1</string>
  <key>provides</key>
  <array>
    <string>cmd:foo-1.0_1</string>
    <string>foo-0_1</string>
    <string>awk-0_1</string>
  </array>
  <key>provides-priority</key>
  <dict>
    <key>awk-0_1</key>
    <integer>25</integer>
    <key>foo-0_1</key>
    <integer>100</integer>
  </dict>
  <key>short_desc</key>
  <string>foo pkg</string>
  <key>version</key>
  <string>1.0_1</string>
</dict>
</plist>